How is a tornado related to a thunderstorm?
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Tornadoes are associated with severe thunderstorms. The strongest of severe thunderstorms, known as supercells, sometimes form mesocyclones. At the base of a mesocyclone can be a wall cloud. If the wall cloud starts to rotate, a funnel cloud and eventually a tornado can form and drop to the ground from the wall cloud.
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Tornado is the most violent of all storms. A tornado, sometimes called a twister, consists of a rapidly rotating column of air that forms under a thundercloud or a developing thundercloud. Tornado winds swirl at speeds that may exceed 300 miles (480 kilometers) per hour. A powerful tornado can lift cattle, automobiles, and even mobile homes into the air and destroy almost everything in its path. Fortunately, most tornadoes are relatively weak, and only a few are devastating. \nScientists use the word cyclone to refer to all spiral-shaped windstorms that circulate in a counterclockwise direction in the Northern Hemisphere or in a clockwise direction in the Southern Hemisphere. The term cyclone comes from the Greek word for circle. Cyclones come in many sizes. Among the largest such storms are hurricanes and typhoons, which may reach 250 miles (400 kilometers) across. \nMost tornadoes are small, intense cyclones. On rare occasions, the winds whirl in the direction opposite that of a cyclone--for example, clockwise in the Northern Hemisphere. \nMost tornadoes have damage paths less than 1,600 feet (500 meters) wide, move at less than 35 miles (55 kilometers) per hour, and last only a few minutes. Extremely destructive twisters may reach 1 mile (1.6 kilometers) in diameter, travel at 60 miles (100 kilometers) per hour, and blow for more than an hour. \nThe United States has the highest incidence of tornadoes in the world. Most of these storms occur in a belt known as Tornado Alley that stretches across the Midwestern and Southern states, especially Texas, Oklahoma, Kansas, Nebraska, and Iowa. However, tornadoes also strike many other parts of the world. Australia ranks second to the United States in number of twisters, and many damaging tornadoes strike Bangladesh. Tornadoes occur most often during the spring and early summer in the late afternoon and early evening. \nA tornado over a body of water is called a waterspout. Waterspouts occur frequently in summer over the Florida Keys. Waterspouts also form elsewhere in the Gulf, along the Atlantic and Pacific coasts, over the Great Lakes, and even over the Great Salt Lake in Utah. \nThe story of a tornado. The majority of tornadoes develop from severe thunderstorms. A hurricane, when it makes landfall, can also generate tornadoes. \nThe most damaging tornadoes form in large, powerful thunderstorms called supercells. For a supercell to form, and perhaps spawn a tornado, several conditions must exist. There must be an adequate supply of moisture to feed the storm. In Tornado Alley, air from the Gulf of Mexico provides the moisture. There must be a layer of warm, moist air near the ground and a layer of much cooler air above. Often, a front (the boundary between two air masses at different temperatures) powers an upward flow of warm air. As the warm air rises, it begins to cool, and the moisture it holds condenses into raindrops. The air stops rising at high levels and spreads sideways to form a characteristic anvil-shaped storm cloud. \nFor a supercell to develop, the winds at higher elevations must differ markedly from those at lower levels in speed, direction, or both. Such a large difference in wind speed or direction is called wind shear. Wind shear makes the column of rising air begin to rotate, forming a broad, horizontal tube of swirling air. As the storm continues, this tube turns on its end, producing a rotating column of air called a mesocyclone. Studies show that most supercells containing mesocyclones eventually produce tornadoes. \nA low, dark, heavy cloud called a wall cloud forms underneath the mesocyclone. Tornado funnels develop out of the wall cloud. \nThe first sign of an approaching tornado may be light rain, followed by heavier rain, then rain mixed with hail. The hailstones may grow to the size of golf balls or even baseballs. After the hail ends, a tornado may strike. In most tornadoes, a funnel-shaped cloud forms and descends from the wall cloud until it touche
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Usually, heavy rain stroms are always accompanied by thunderstorms.  The worst storms occur when warm, moist air meets with cool, dry air.  The greater the difference, the greater the thunderstorm and rainfall.
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The wind starts moving in two different directions from high pressure to low  pressure, causing a tornado this is a\nsmall sample of southeast Kansas weather every Spring.
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A storm becomes a supercell about five seconds before my crazy tornado freak father calls my house and says, "you better get your a$$ over here, there's a supercell heading this way and we have to get into the cellar."  The supercell has been downgraded to a thunderstorm about five seconds before my crazy tornado freak father calls my house and says, "you're damn lucky that storm didn't spawn any tornados, your little a$$ would've been blown away. "
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What causes tornadoes?\n\nThunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes.\n \nDuring the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours. \nAlong the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes.\n\nTornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes onshore.
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According to answers.com\nhttp://www.answers.com/tornados%20\n\n"Tornadoes develop from severe thunderstorms, usually spawned from squall lines and supercell thunderstorms, though they sometimes happen as a result of a hurricane. They are believed to be produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly, though tornadoes over water (waterspouts) frequently are observed forming in the absence of convection or apparent strong surface temperature differences. Tornadoes, lightning, and sometimes hail are associated with thunderstorms. Many tornadoes appear at the tail end of mesocyclones. On weather radar screens, a characteristic "hook echo" marks the area where tornadoes are likely to exist."\n\nThe reference will give good idea for how do tornados form
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Technically yes there must be a storm of some kind... But the tornado isn't usually in the center of a storm usually its on the southwestern corner or side of the storm and it might not even be raining in that part of the storm, making it appear that its not in the storm.\n\nTo have a tornado you need these 2 things....\n\n1. A convective rain shower/thunderstorm\n2. Winds in the upper and middle layers of the atmosphere running Perpendicular to eachother, or the winds in the lower and middle layers. ( basically wind blowing in 2 different directions at different altitudes of the storm, like at the ground the wind be blowing from the west and up about 10,000 feet the wind blowing from the south.)\n\nThats really about it... Those dont really mean that you will get a tornado, it just that those things must be present to create a tornado.
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Tornadoes are formed by the clashing of drasticlaly different masses of air known as fronts.  A high pressure front meets with a low pressure front and the difference in their temperatures and pressures creates a strong wind that can form into the funnel clouds known as tornadoes.  This is not to say that all colliding fronts will yield tornadoes, but there are certain elements that can increase the likeliness of a tornado forming.  The swath of the mid-west United States from N. Texas / Oklahoma through Kansas / Nebraska and into Iowa / Missouri is known as "Tornado Alley" because there are more tornados per year in this area than anywhere else in the world.
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You're right--lightning is just a spark. BUT it's a spark caused by massive voltage. Voltage can be defined as a difference in electrical potential between two points, with one point having a high negative electrical charge, the other a high positive electrical charge. This difference in electrical potential can become enormous when a thunderstorm gets going full force.\n\nRain and/or hail in the storm clouds are violently rubbed together by high-velocity winds, thus building up a net negative electrical charge (abundance of electrons). On the ground below the clouds no such action takes place, thus the ground has a net positive electrical charge (deficiency of electrons).\n\nEven though air is a very good insulator, the difference in electrical potential (voltage) between the storm clouds and ground can quickly reach as high as 500,000 volts, more than enough to produce a lightning "spark."
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A supercell is an organized thunderstorm that contains a very strong, rotating updraft. This rotation helps to produce severe weather events such as large hail, strong downbursts, and tornadoes. Supercell storms are usually isolated from other thunderstorms because it allows them to have more energy and moisture from miles around. These storms are rare, but always a threat to life and property.
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A tornado WATCH means conditions are right for a tornado to occur.  A tornado WARNING means a tornado has been sighted in the posted area or is visible on radar.
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You can't tell by just stepping outside, unless you see a funnel cloud or something.  Tornadoes are usually created from cold fronts or occluded fronts.  You have very warm air ahead of the front, and very cool air behind it.  The difference in the air temperature pushes air up, creating an updraft.  Bottom line, tornadoes are caused by warm and cool air in the same area.  And you have to have very strong winds in the upper atmosphere.  70 to 80 mph.  That helps with the updraft.
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According to the National Severe Storms Laboratory, central Kansas receives the most thunderstorm activity.  Unfortunately, those storms come with tornadoes.  Florida gets the most lightning, sometimes with thunderstorms but that also means hurricanes.
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Dave in NC answered it perfectly.  Ill just add, if you are asking why tornados form over land- The rapid increase in heat needed to cause formation of tornados generally occurs only over land, since water modulates changes in heat.  However, sometimes they do form, but they are called waterspouts, and usually occur near hurricanes.
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In the US Midwest, tornadoes are usually formed when a cold, dry air mass moving south from Canada collides with a warm, moist air mass moving north from the Gulf of Mexico (or Baja California, or the Atlantic, or what have you). \nThis produces large thunderstorm clouds known as supercells, which can produce heavy rains, hail, and tornados. \nTornados are formed in areas of the supercell called mesocyclones; which draws air into the storm. When you hear meteorologists talk about the "hook echo" on the radar, they're pointing out the mesocyclone. The best simple explanation (so far) for why the tornado starts to rotate is the phenomenon of vertical wind sheer, where wind changes direction at different elevations.\nSo, if you have access to a weather radar; you can see hook echos like the one below. If you have a wind velocity view (some do, some don't), look for a large difference in wind speed and direction around that hook echo.
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A thunderstorm, or an electrical storm, is a form of weather characterized by the presence of lightning and its attendant thunder. It is usually accompanied by copious rainfall, hail, or, on occasion, snowfall in the winter months (which is sometimes known as thundersnow).\n\nThunderstorms can happen anywhere in the world, with the greatest frequency in tropical rainforest areas, where they may occur nearly daily. In temperate regions, they are most frequent in summer, though can occur in cold fronts at any time of year.\n\nThunderstorms form when significant condensation, resulting in the production of a wide range of water droplets and ice crystals, occurs in an atmosphere that is unstable and supports deep, rapid upward motion. This often occurs in the presence of three conditions: sufficient moisture accumulated in the lower atmosphere, reflected by high dewpoint temperatures; a significant fall in air temperature with increasing height, known as a steep lapse rate; and a force such as mechanical convergence along a cold front that will focus the lift.\n\nThunderstorms have had a lasting and powerful influence on early civilizations. Romans thought them to be battles waged by Jupiter, who hurled lightning bolts forged by Vulcan. Thunderstorms were associated with the Thunderbirds, held by Native Americans to be a servant of the Great Spirit. In more contemporary times, thunderstorms now have taken on the role of a curiosity. Every spring, storm chasers head to the Great Plains of the United States and Canada's prairie provinces to explore the visual and scientific aspects of storms and tornadoes.\n----------------------------------------------------\nU can also find more here: http://www.nssl.noaa.gov/edu/storm/