This document discusses occlusion and provides definitions and classifications of different occlusion types. It begins by defining occlusion and describing it as a complex phenomenon involving the teeth, periodontium, jaws, TMJs, muscles and nervous system. It then discusses the different types of cusps on posterior teeth, arrangements of teeth including cusp-fossa and cusp-embrasure occlusion. It also describes imaginary occlusal planes and curves, centric relation and occlusion, centric and eccentric contacts. It concludes by discussing Andrew's six keys to normal occlusion.