Think of it as buying a house. The upfront purchase cost covers the deeded property, but you will still have additional yearly costs to cover insurance, utilities, maintenance, taxes, furniture, etc.
DVC works the same way. The initial cost covers the ownership and the annual maintenance fees cover all of the additional costs of ownership.
Here is a link to the history of these annual fees since 1991 when DVC began. It is from the DVC Resource Center noted at the top of each DVC forum.
You buy points for $XX.XX per point, your receive these points every year on your UY date. Using a point chart, you trade the points for a room at a DVC resort. The number of years that you can do this is determined by which resort you purchased and it's contract termination date.
It addition to yearly dues that increase around 3% per year, you need to consider the yearly cost of transportation, food, admission and extras which also increase each year.
And even though it's suggested by the posts above, I just wanted to say -- there is no such thing as a $12,000 membership in DVC really.
You are buying a real estate interest, and depending on where you buy, how many points you buy, and where you buy from, your price could range from a few thousand to many thousands. I assume you are looking at an existing contract though resale that is priced at $12,000.