EF is the standard autofocus mount for all Canon EOS bodies; 35mm film, full-frame digital APS-H 1.3x crop digital (i.e. the 1D series) and APS-C 1.6x crop digital bodies. They feature a red dot on the lens barrel to indicate where they should be aligned before mounting on the camera.
EF-s is the line of lenses designed for use only on APS-C digital bodies (500D, 60D, 7D, etc). They cast a smaller image circle and are usually lighter and smaller than their EF counterparts. They feature a white square on the lens barrel to indicate where they should be aligned before mounting on the camera and will not mount on full frame bodies.