The World's Crustal Plates
The Earth's outer layers are organized into about a dozen great pieces, called lithospheric or crustal plates—although "shells" is a better name when you realize that they're all curved, not flat. The interactions of these plates, and how they recycle most of the lithosphere over the course of geologic time, are what plate tectonics is all about. In addition to these major plates, there are another dozen minor ones or microplates (see the full list, with their sizes).
Beneath the plates is the upper mantle. The top part is softer than the the crust above or the mantle below, and that layer is what allows the plates to move without stirring up the deep mantle. To learn more, see Plate Tectonics in a Nutshell.
See a larger version of this image here. The image comes from the fine online pamphlet "This Dynamic Earth" from the U.S. Geological Survey.
