Plates move because convective currents rise through the plastic mantle, as heat interior to the Earth (from radioactive decay) tries to escape. Where the flow is up, it spreads the crust (mid ocean rises) and where it flows down, subduction zones are created. There are two kinds of crust: continental which floats higher on the mantle, and oceanic.

The word tectonic refers to motion. Where plates are moving against each other, quite interesting things occur: Mid-Ocean Ridges. Spreading centers where the mantle is up-welling. This is where new crust is created. (Middle of Atlantic Ocean)
Subduction Zones. Where crust is being lost, usually oceanic crust sliding below continental. Occurs a little at a time, with sudden motions and earthquakes. Usually aligned with mountains (Nazca Plate into the S.A. plate, creating the Andes Mtn)
Transform Faults. Where two plates are sliding against each other, though no crustal material is lost. Again, lots of earthquakes but no mountain ranges (San Andreas Fault)