It's a timeshare and it is basically a scam for most people. Basically you're buying time to stay at the place that you can technically use at any time year round as long as you book it in advance. The problem is that all of the days that most people go on vacations (spring break, summer, holidays, etc) will already be booked by the 500 other people who also bought the same thing. So really the only times you will actually be able to use the place are when no one else wants to be there.
If you have the ability and desire to drop everything and vacation there on a random week whenever it happens to be available then it can still be a good deal for you, but most people have obligations like work, school, or family that limits when they can just drop everything and leave to spend time at their timeshare.