Here's an example:
1) You buy 150 points with Sept. use year (every Sept. 1, you get your allotment of 150 points) at SSR
2) You want to go to WDW the month of May for 5 days (Sun-Thu) in a 1 bedroom. Here's the breakdown of points for that week:
OKW - 100 points; BWV - 140 (preferred to simply example)
BCV - 140 points; VWL - 140
SSR - 125
3) You've read these boards for months now and you've become frugal with your points

. You decide that OKW looks great at 100 points!!
4) You check the calendar and find that you are within the 7 month window to make the reservation.
5) Call Member Services. You tell them the dates you want to go and the resort you would like to stay in. They check... yep OKW had a 1 bedroom available. You say, "I'll take it!!" The CM asks you the names of your party, any requests, and then tells you a conf. # and the cancelation policies.
6) You receive an envelope that makes all DVC members hearts race. You open it up and there is this beautiful confirmation letter. You are on

and you start to mentally pack your bags for the trip!!
7) What to do with the left over 50 points??? You can bank them for the next years vacation, rent them to someone, or go back to WDW for a short trip!! It's up to you!!
Like the other members said, if you want to use your points outside the DVC resorts (i.e. exchange your points for a week just about anywhere), there is a small fee for them to process your request and find a place for you.
We bought 200 points at BWV in 2000. We banked out points that first year because we were not going back to WDW within the year. We had a baby in 2001 and used the 200 points we had banked for an exchange. It cost us $75. We banked the 2001 points. In 2002 we went to FL for 2 weeks - 1 week at OKW and 1 week at VB. We've gone back to WDW 2 more times and another trip planned for this Sept. We have never stayed at our home resort! We love OKW and try and stay there!!
I kind of look at the maintence fees at HOA (homeowner's association fees). Some subdivisions have lower and some have higher!!
Good luck!!