It is a point based system.
There are different types of rooms, at different resorts, each requiring different point values for a stay, and those point values differ on the times of year. Friday and Saturday nights are more points than week nights.
Easiest way is to go to dvcrequest.com and use the points calculator to get an idea.
However many points you buy, you get that many every year (on your use year date)...you can bank 1 year, and borrow one year, so in technically you can have up to 3 times the amount of points you purchased available to you.
For Example, I just bought 150 points at Grand Floridian with an April Use year.
I have 150 points from this year available to me, and I can borrow against next years. If I do not use any between now and next april, I can then borrow my 2015 points, giving me 150 from 2013(banked-unused), 150 from 2014(current) and 150 from 2015(borrowed) giving me access to 450.
I am planning on going in December, and I need more than 150 for what I want, so I will use all this years, and I will be borrowing some from 2014.
You can pay up front, or disney does offer financing options (at a not so friendly interest rate)