An average elephant has two tusks, which are technically extended incisors, and then four molars with one molar conforming to each half of the upper and lower jaw respectively for a total of six teeth. Elephants wear through their molars over time and go through an average of six sets of four over the courses of their lives.
Tusks are used for rooting in the dirt and mud as well as for defense and for contests of strength and dominance. Bull elephants may duel one another using their tusks and can seriously injure or even kill each other when their confrontations over mates and territory become heated during mating season.
Molars are flat and very large and are used to grind up tough vegetation. Because elephants eat such a vast amount of roughage even in a single day, they wear through their molars and cause them to grow smooth and useless in chewing. New molars replace the old teeth until the elephant stops producing replacements, at which point it may starve or grow ill.
The molars of an elephant actually travel along the jawbone because they do not have a socket to fix them in place. Newer molars displace old ones and take their spots along the jaw.