A refinement of Alfred Wegener's continental drift theory.
The Earth's outermost layers are the crust (where we live) and the upper mantle. The crust is 35-100km thick on land, but only 5-10km thick in the oceans.
Together the crust and upper mantle form the lithosphere.
Below lies the molten asthenosphere. Slow convection currents in the asthenosphere create volcanic action in the deep fissures of the sea. New material is forced into the lithosphere by this action, and the lithosphere is forced outward.
The lithosphere is broken into around a dozen large and small sections called plates. Differences in the composition and density of the rock in the crust makes the continental plates more buoyant than the sea plates. (Sea plates are composed of moon-rock-like basalts.)
Pressure on the plates at the boundaries results in them moving about. They may drift apart (extenional), where new material is created at mid-ocean ridges by an upwelling of lava from below; collide (compressional), where subduction forces crust back into the magma layer ocean trenches, or grind alongside each other (transform). The San Andreas fault is a transform type of fault where the Pacific plate grinds against the North American plate.
Plates move constantly but travel only a few centimeters every year.
Major plates: The African, Antarctic, Eurasian, Indian-Australian, Nazca, North American, Pacific, and South American plates.
Smaller plates: the Anatolian, Arabian, Carribean, Cocos, Juan de Fuca, Philippine, and Somali plates.
Plate names from http://www.solarviews.com/eng/earthint.htm#plate