DVC is not like other timeshares, where you might get a certain room or room type for a certain week every year. It's more flexible, but also more complicated: "joining" the "Club" means buying a real estate interest that is represented as a certain number of points. Those points, simply put, are what you're buying, and they can be used toward any room type at the resort you buy into (your "home resort"), at any time of year, provided it is available. A three-bedroom grand villa costs more points than a studio, and the week of Christmas costs more points than the last week of January. You have to own a larger interest in the resort in order to book larger rooms or book during peak times of the year, and this larger "share" of the resort is represented by owning a higher point count. There are a finite number of points in existence for each resort; the only way Disney can sell more points is to either buy some back and resell them or introduce a new resort, which they do every two years or so.
This means that even if you own enough points to theoretically book, say, a standard view studio at Boardwalk for the first week of September every year, if you aren't on the ball other people can book up all the studios that are available, because everybody that owns points at Boardwalk has access to the same rooms. Even people who might normally book a two-bedroom for a week every year might decide to go for a studio for three weeks that year and beat you to it — their points aren't "tied up" in the two-bedrooms, even if that's what they bought enough points for and what they normally book. If you miss out, your points don't go away — you can use them for a higher class of room (for fewer nights), you can go a different time of year (which might be more expensive), or, once you're within 7 months of your desired date, you can use them to try to book at another DVC resort.
This is why it's good to be at least a little flexible in what types of rooms you want, especially if the room type you're looking for is more in-demand (and the cheapest rooms are the ones that are in demand). If you have your heart set on staying for a week each year, for example, I wouldn't buy just enough points to get the value studio at Animal Kingdom in low season for a week, because you might not be able to get that every time. There will be a lot of other people with the same idea who will be rushing to book the same room type 11 months out on the dot.
If you want the convenience of a having a set vacation week each year and don't want to have to worry about whether or not you will be able to get the room you want, Disney does offer a "guaranteed week" option at some of their newer resorts. This option does cost more but you will always be guaranteed to get the exact room class you want on the same week or weeks each year (though you can always elect to instead use your points to book a different type of room or different days, or book at a different resort, etc.). If you choose this option your reservation will automatically be made for you each year before the resort is open for booking to other guests, even guests who have an ownership interest ("points") at that resort. However, this option is not available at older DVC resorts, such as Animal Kingdom Lodge.