You pay upfront (or finance) to join, which is based on the number of points you purchase. Currently, you must purchase a minimum of 150 points, and they are either $98 per point or $101 per point (July 1 for everyone, I believe). Then, you only pay an annual maintenance fee, which is priced per point. Each resort has a different maintenance fee cost - I believe they range from $3.75 or so per point to $4.50 per point. For 150 points, that comes out to between $560+ to $675. We have 240 points at Saratoga Springs, and we paid about $900 for this year's maintenance fees.
You get your points every year, under a use-them-or-lose-them system. However, you can bank points for upto 1 year, and borrow from your next Use Year. You can bank or borrow all of your points - ie. If you had 200 points, you could bank all 200 in Year 1, then in Year 2 borrow all 200 from Year 3, to give you 600 points to use in Year 2. Banked or borrowed points must be used, or they are lost.
Hope this wasn't too confusing, and I hope it helps! Feel free to ask more questions, we'll answer as best we can. However, booking a DVC tour during your next stay will answer your questions, also.