Rockets are commonly used when it is necessary to carry all the fuel a vehicle needs (such as in outer space) and there is no other substance (land, water, or air) that a vehicle may push itself with.
A chemical [rocket engine]? may use solid fuel, like the [Space Shuttle's SRBs]?, or liquid fuel, like the Space Shuttle's main engines. A chemical reaction is initiated with the fuel in the combustion chamber, and the hot gasses are forced out the rear of the rocket. This causes the rocket itself to be thrust forward.
Nuclear-powered rockets have also been developed, but never put into use. Public opinion and various international treaties prohibit their development and use, even though their performance would make interplanetary missions much more practical.
Rockets were first developed by the Chinese a long time ago (how long?), using gunpowder. These were initially developed for entertainment, the precursors to modern fireworks. It was not until much later that they were used as weapons in warfare.
Rockets range in size from tiny models purchased at a hobby store, to the enourmous [Saturn V]? used for the Apollo program.
In military terminology, a rocket is generally solid-fueled and unguided. These are typically fired by ground-attack aircraft at fixed targets such as buildings. Whereas a missle? can be either solid or liquid-fueled, and has a [guidance system]?.
See spacecraft propulsion for more types of rocket engines.
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