Force is what causes an object to undergo acceleration: to change its velocity, rotation, shape, pressure, and so on. The SI unit for it is the Newton. Force is a vector, consisting of a magnitude and direction. The resulting acceleration is in exact linear proportion to the magnitude of the force applied, and in the same direction.
There are various types of forces, however, all of them are consequences of fundamental forces: gravitational force, electromagnetic force and the strong and weak nuclear forces. The electromagnetic and weak nuclear forces have already been united into a single electroweak theory, which governed the time when the universe was very hot and dense, before the two forces broke symmetry. The strong nuclear force has yet to be united with electroweak in a Grand Unified Theory (or GUT). The holy grail of physics is to unite this GUT with a quantum theory of gravity to produce a Theory of Everything (or TOE).