http://disneyvacationclub.disney.go.com/
It is similar to a timeshare in many ways in that you "purchase" an interest in a resort (e.g. Aulani, Old Key West, Animal Kingdom Villas, etc.) What you are buying is actually just a yearly allotment of points that you can redeem for lodging. It is unlike a timeshare in that it expires 60 some-odd years after the particular resort you are buying interest in was opened for sale.
You pay for the points up-front and then have a yearly maintenance fee on those points. The advantage is that, if you vacation with Disney a lot, you can save a lot of money in the long run. There are lots of ancillary factors like buying direct from Disney vs. resale, the economy of using points for things outside DVC resorts (like
DCL), renting points, how often you want to visit a Disney resort, banking and borrowing points from year to year, etc.
DIS has a good FAQ at
http://www.wdwinfo.com/disney-vacation-club/faq.shtml