Once formed, a thunderstorm exists for one purpose: to destroy itself. It's mission is to stabilize the atmosphere, which means working against the very conditions that produced it. A thunderstorm can be thought of as a huge heat machine. It is fundamentally nothing more than a mechanism to pump warm, moist air from the lower atmosphere into the upper atmosphere, and draw cold, dry air from the upper atmosphere down to the lower atmosphere. It thus works to weaken the temperature gradient that caused it in the first place, by cooling the boundary layer and warming the air above it. Once the gradient is sufficiently weakened, the boundary layer air is no longer bouyant and ceases to convect upwards. This cuts off the inflow of the thunderstorm, so once the storm is finished dumping its moisture, it ceases to exists. While the thunderstorm is active, a number of processes are occuring that produce the weather we normally associate with thunderstorms: strong winds, hail, lightning, heavy rains and tornados.
At the core of the storm, warm moist air is being sucked upward in the updraft which is usually near the rear (typically southeast) of the storm. This is called the inflow, and this can cause strong winds around the rear of the storm as nearby air is drawn into the inflow. If inflow is strong enough and some vorticity is present (usually caused by the upward tilting of horizontal rotation (called helicity) which the recent VORTEX research project has associated with outflow boundaries and other mesoscale anomalies) then the updraft column begins to rotate, which lowers the pressure in the core of the updraft, which further strengthens it. The low pressure causes the moisture in the rotating air to condense and form a cloud called a funnel cloud. If this continues long enough, the rotation extends downward until it reaches the ground, at which point a tornado has formed. Ascending air in the updraft cools until it condenses and is thrown out the top of the storm whereupon it spreads outward. The water contained in this air is then dumped downward, especially immediately in front of the updraft area, where the heaviest rains occur. The is called the outflow of the storm. Sometimes the outflow is accompanied by a strong downburst (either a microburst or macroburst, depending on its size) of air that, upon striking the ground, spreads outward along the surface as strong winds, the leading edge of which is termed the "gust front". These winds can exceed 100mph and can be as damaging as some tornados. Often bow echo (a certain kind of multicell storm) storms are characterized by strong gust fronts or downbursts. If you experience a gust front, you'd better take shelter because in a minute or two you'll be hit with the leading edge of the rain outflow - the heaviest rains of the storm! The upward and downward motions within a thunderstorm causes air to rise above and fall below the freezing point, upon which moisture changes from water to ice and back to water again. There is something about this process that tends to create an electrostatic gradient in the thunderstorm cloud (called a CB for short). The top of the CB becomes positively charged and the bottom negatively. If the gradient is strong enough, electrons from the bottom of the cloud will discharge to the top in the form of lightning (called IC lightning or intracloud lightning). A negatively charged cloud base repels electrons on the surface and thus causes a positively charged surface. A discharge from the cloud to the ground is called CG lightning (cloud-to-ground lightning). Further, the freezing and melting of particles causes them to grow as layers of ice are added to them with each ascent above the freezing level. If allowed to rise and fall repeatedly enough times, eventually its weight will cause it to drop to the ground as hail. Thunderstorms have been known to produce hail as large a grapefruit! More financial damage is caused by hail than any other weather (even tornados) from a thunderstorm, mostly because of the extensive crop damage it produces in Midwest farms.