The rear is the back end of something, like the rear of a school bus or the members of the marching band who stand at the rear of the parade.
At the other end of something's front is its rear, whether it's a car or a line of people standing outside a theater. You can also use rear informally to mean "bottom" or "hindquarters," as an adjective meaning "toward the back," or as a verb: "The horses started to rear up on their back legs." The verb comes from a Germanic root meaning "to raise," while the noun is rooted in the Latin retro, "behind."