
The system is actually a simple concept on the surface. You are essentially buying into a timeshare, but unlike traditional timeshares, you are not locked into any particular week of the year. Rather, you can take a vacation whenever you wish at any of the Disney resorts for as long or short as you would like, depending upon availibility and the number of points you own. Your portion of this timeshare is expressed as annual points, thus why Tink10 gave you the example of 150 points, the minimum new buy-in at SSR. So, when you make reservations for a trip at a given resort, it will cost you a certain number of points each night with peak seasons and certain resorts costing more.
How many points you buy determines your final cost--150 is simply the minimum. That statement begs the question: "How many points are enough for me?" Only you can determine that based on your travel habits, intentions, and finances. Other factors that weigh on the decision are when you travel (peak times use more points), how long you stay, and what sort of arrangements will you need (studio, 1BR, 2BR, or 3BR grand villa).
Another question that arises is "Is DVC a good option for me?" Again, it all depends upon a few factors, not the least of which is will you travel to WDW (or its travel partners) at least once every other year? If you can make the WDW trip just once every two years, most people would tell you that in the long run the DVC will pay for itself and then some. However, do not look at DVC as an investment, merely as prepaying vacation accomodations for the next 50 years. In case it's not obvious (it wasn't to me at first), DVC membership allows you to not have to worry about paying for accomodations at a Disney resort every time you visit. You are paying up front. DVC membership does not include food, park tickets, or other attractions such as night club admissions. They usually offer a 10% discount on an AP ticket, but unless you visit WDW more than a couple of times a year, they're not cost effective. You do get free valet parking at your resort, though that's not much of a benny in my opinion.
The resort into which you buy is considered your home resort which allows you to book a room 11 months in advance; if you choose to stay in a resort that is not your home resort, you have to wait until the 7-month point. For some people, they consider it best to buy where you want to stay most often. For example, if you love the charm of VWL and plan to stay there every time you visit, it might be worthwhile to buy a resale into VWL so that you get that 11-month out window vice 7-month. If you are flexible with your travel plans and enjoy all the resorts (such as my DW and I), then the 11-month out advantage shouldn't be a huge selling point. Besides, SSR is what the DVC offers right now; resales can take much longer, especially the more popular ones such as VWL or BCV.
One last item of importance is the banking and borrowing of points. Without going into too much detail, just realize that you can borrow points from a year ahead, or you can skip a trip one year and bank those points to save until the next year. In fact, you could bank one year's points, borrow the next year's, and add both of those to your current year for a huge family gathering if you wanted. Each option carries some restrictions, but they're fairly easy to follow. Furthermore, you can rent your points to someone else, give the points as a gift, or even pass them along to your children or other relatives in a will (keeping in mind everything expires in 2054).
Tink10's figures and comments were correct re: financing, point costs, and other economic considerations. The only missing element is that you also have to pay maintenance fees each year, currently around $3.50-$4.00 per point (e.g., 150 points x $4 = $600). Even once you pay off your loan, say in 10 years, you will still have to pay the yearly fees that have been rising over the years, though usually no more than inflation. Well, hopefully, I've given you some insight into the basic fundamentals of DVC ownership. Only you can decide if it makes good financial sense for your situation. For more detailed info, I would strongly suggest you contact DVC and speak to a guide. Better yet, if you can wait to go on a tour while at WDW, it's great to see everything first hand. The guides don't use high pressure sales tactics at all--it's just a very informative session. Good luck on your decision.