A rocket can be a missile, spacecraft, aircraft or other vehicle which obtains thrust by the reaction of a rocket engine by which ejection of a fast moving fluid from a controlled and concentrated exhaust creating a thrusting action.
However, most spacecraft today are propelled by exhausting a gas from the back/rear of the vehicle at very high speed through a special cone shaped supersonic exit called a de Laval nozzle.
Chemical rockets create their exhaust by the combustion of rocket propellant. The action of the exhaust against the inside of combustion chambers and expansion nozzles is able to accelerate the gas to hypersonic speed, and this exerts a large reactive thrust on the rocket (an equal and opposite reaction in accordance with Newton's third law). Chemical rockets store a large amount of energy in an easily-released form, and can be very dangerous. However, careful design, testing, construction and use minimise risks.
Rockets, in the form of military and recreational uses, date back to at least the 13th century. Widespread military, scientific, and industrial use did not occur until the 20th century, when rocketry was the enabling technology of the Space Age, including setting foot on the moon. They are also used for fireworks, weaponry, ejection seats, launch vehicles for artificial satellites, human spaceflight and exploration of other planets. While comparatively inefficient for low speed use, they are very lightweight and powerful, capable of generating large accelerations and of attaining extremely high speeds with reasonable efficiency.
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled a rocket engine.
All current spacecraft use chemical rockets (bipropellant or solid-fuel) for launch, though some have used air-breathing engines on their first stage. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control.
Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north-south station keeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have experimentally used ion thrusters (a form of electric propulsion) to great success.