It is basically a timeshare (although Disney hates using that term because there are A LOT more options than a typical timeshare). We love being members as it gives us a lot of flexibility with our vacations. You can go to Disneyvacationclub.com to learn more about it.
You can also go a little further down on the Disboards main forum page and you will find the DVC forums, the members there would be happy to answer any specific questions you might have.
DVC- Disney Vacation Club, is Disney version of a timeshare. You purchase a contract which gives you a certain amount of points(which replenish every year). You can then use these points to book DVC resort rooms(or regular WDW resort rooms if you wish). Check out this link.http://www.wdwinfo.com/disney-vacation-club/index.html#hm-dvc-nav