In liquid propellant based rocket engines, the fuel and the oxidizer are liquids, which have to be stored separately. The fuel and liquid oxygen are stored in separate tanks from which they are pumped through injectors into a combustion chamber. The injectors mix the fuel and oxidizer thoroughly so that the fuel burns properly. The burning of the fuel in the combustion chamber, produces gases at a high temperature and pressure. The gases are then forced out of a narrow nozzle at the bottom of the rocket. This generates the thrust necessary for the rocket to move upward.
In addition to the weight of the propellants themselves, a liquid propellant based rocket must also carry the additional weight of the storage tanks, pumps, valves, injectors and piping. This increases the entire weight of the rocket.
The additional components, i.e., the pumps, tanks and injectors also means that the design of liquid propellant rockets is more complicated than solid propellant based ones.
However the liquid propellant rockets are very important because they are easy to control. The thrust, and therefore the speed, produced by a rocket can be easily controlled by controlling the amount of propellant that is pumped into the combustion chamber.