(image from book Structural Geology of Rocks and Regions by G. H. Davis and S. J. Reynolds page no 614 chapter 10)
The principal forces that control the plate movements are:
Ridge Push
At the spreading centres the newly generated lithosphere is hot and buoyant. Thus the topography becomes a Ridge. A gravitational head is thus created and Ridge Push force is applied to the separating plates.
Slab Pull
As the plates separate from each other from the ridge the lithosphere gradually becomes cold and its density increases. The density contrast between the cold slab and the hot mantle gives rise to negative buoyancy which in turn generates the Slab Pull force.
Slab Suction
This is a force which drives the overriding plate into the trench at a faster rate.