The principle of rocket propulsion follows Newton's famous third law of motion, which states that to every action there is an equal and opposite reaction. A rocket engine travels by expelling mass in one direction while at the same time using the reaction to move in the other direction.
Though the principle seems simple in theory, in practice, many other factors need to be considered to make a rocket move properly. Force is mass multiplied by acceleration, so the weight of the rocket and its fuel are important considerations in determining thrust. Since fuel is carried on the rocket, a high escape velocity is desirable, as the rocket uses more fuel to achieve thrust at low velocity than high velocity. Once the rocket picks up speed, less fuel is needed. Rockets needed to lift people or objects into space are so large because most of the bulk is taken up by fuel.
Both solid and liquid fuels are used in rockets. Solid fuels have the advantages of safety, simplicity and low cost, but once begun, the engine cannot be stopped or restarted, and the thrust cannot be controlled. For these reasons, solid fuel engines are used for booster systems or missiles. Liquid propellant engines are more complicated and volatile, but they have higher exhaust velocities and are easier to control, start and shut down in an emergency.