WALL-E is our main character, a little trash compacting robot, whose name comes from his model—Waste Allocation Load Lifter: Earth Class.
The "Waste" is all the garbage that humans have left behind, and he's "allocating" it into giant piles as tall as skyscrapers by compacting them into little cube-sized "loads" and "lifting" one atop of the other like building blocks. And where is he doing this? Earth, of course.
Humans have trashed the place, and it's this robot's mission to clean it up. There were actually many identical WALL-Es originally, but most have powered down by the time the movie starts and WALL-E is the only one to survive. This makes us wonder if he's more than just a lowly robot—perhaps he's a creature capable of human emotion?