In Freemasonry, what is the Great Work?

I've been reading up on freemasonry a lot, I have an opportunity to join and am wondering if its the right fit for me. I like self-improvement, philosophy, and esoteric knowledge, but I've never been much of a group person. One thing that I keep coming back to is I've seen or heard the reference to the Great Work a number of times, but haven't seen any clear explanation on what it is, or what the end goal of it is. I have my own ideas on it, but I would like the guidance of another adept that has a clearer vantage point than I.