Curated OER
National Park Service: What on Earth Is Plate Tectonics?
An easy-to-understand explanation of what plate tectonics is. Contains information about the inner parts of the Earth, the difference between the three types of plates, and numerous maps and illustrations.
Other
University of Delaware: Plate Tectonics
This site is a very simple overview of plate tectonics. It includes a map of all the major plates and a diagram of the layers of the earth.