It's an audience interpretation thing. Nobody really has a "correct" explanation for what it is, but my theory is basically

http://www.youtube.com/watch?v=f_J5rBxeTIk

I think the ending relates to Dr. Bowman, the main char being the next stage in evolution. He finds the orbiting monolith, and it communicates with him and shows him an allegory of what his "human" life would be like; growing old, dying, and then it offers him a chance to evolve to be the first in the next step of what man will become. He then touches it like the cavemen did at the beginning of the movie where upon touching it they rose above basic instincts and began to have complex thoughts. When he turned into the cosmic fetus thing floating over Earth, he's almost "godlike", and the movie leaves the audience with a very open ending as to what this new being will do or may be capable of. Hope that helps, I had to do a report on it in a cinema appreciation class.