There are DVC forums on DIS which will have a lot more information, but the basic idea is that you have two sets of costs
1) the cost to buy a certain number of points upfront - you can pay cash, use a credit card or finance this cost (i do not know how long they will let you finance for) I think current costs through Disney are a little over $100/point - you can buy resales for less.
2) the yearly dues/fees associated with those points (this fee depends on which resort you buy in at - SSR is a little over $4/point this year).
You can then use your points each year at various DVC Resorts and other hotels (i.e. we are using points at GCH this month at 24 points per night for a standard room). (We figured with prorating the initial purchase cost that each point "costs" around $10.00 - so each night at the GCH would be about $240 vs. rack rate of $340)
As a general rule of thumb, if you go to WDW once a year or maybe every other year and stay at a deluxe resort, DVC may save you a lot of money.
There are some other perks such as annual pass discounts, etc.