Thermal convection is a process by which heat is transferred via an object that is being heated. This is most commonly seen in liquids and gasses, and can be easily demonstrated using air as an example. It represents one of the major ways in which heat moves, with conduction and radiation also being common ways to transport heat.
Scientifically speaking, coldness in not quantifiable; there is simply more heat or less heat. Therefore, thermal convection is not simply about hot objects moving. It is about the way temperature as a whole seeks to moderate itself, either to its surroundings, or to the objects that cause it to become heated. By design, the movement of warmer and cooler gasses and liquids causes an overall moderation, though a perfect balance may never be achieved.
The principle of thermal convection can easily be seen by opening the door to a heated oven. When the door is opened, a rush of hot air immediately shoots upward from the oven. If one was to place a small flag at the base of the oven, the flag would wave in the direction of the oven as cool air fills in as the hot air rises. When that cooler air is heated, it will also rise.
The air that is rising displaces the cooler air above it, often forcing it to the side, and eventually downward. That air will remain at that lower level until it is reheated, and begins to rise again. This heat flow process repeats itself until the heat source is neutralized, and the temperature throughout the area is consistent. If that does not happen, the process will continue indefinitely.
The process is also seen in water, and is not always helpful. For example, if the surface of a lake cools quickly, the water underneath, which is warmer, pushes up. With this upwelling may come dead matter, such as algae, that had been slowly decaying on the lake bottom. Being exposed to air and sunlight, the decaying matter increases its rate of decomposition, and could starve oxygen from the lake. Thus, in this example, thermal convection is responsible in an indirect of way of great harm to a living ecosystem.
Despite the preceding example, thermal convection is usually considered to be a helpful phenomenon. Most ovens and furnaces run on principles associated with thermal convection, thus making the lives of most people more comfortable. Also, temperature moderation in an ecosystem is often very helpful to the forms of life that live in the system. It is also one of the major driving forces in weather.