As you can tell by the responses, it is important to do some research before investing in a timeshare. In general, in a fixed week unit, you always have ownership to that week. For example, if you own unit 500 on week 21, you will stay in unit 500 on week 21 every year unless you make a decision not to use the week. This is true even if the resort has converted into a points-based system where you still own the week but are put into a point system where you are given so many points for the week you own. To use the unit that you own, you give the points-based system some notification that has been specified that you wish to use your unit. For RCI Points, a points-based system, this is 13 months before you wish to use your unit. So if you own unit 500 on week 21 and it is 2007, you can make reservation for unit 500 on week 21 in 2008 during week 17 in 2007. Failure to provide this notification is taken to mean that you will not be using the unit and as such, it will not be waiting for you and you will need to make a reservation which will probably be in another unit. Fairfield points also fits into this category, at least the initial conversion from what I understand.
A true points-based system such as DVC or Hilton (I believe Sunterra Sun Options also fits this category as you give up your week to be given x amount of points for access to units that are in a trust), there is no ownership of a specific unit and as such, its the luck of the draw if you get the same room.
Confused? Probably. The best thing to do is investigate. Short answers really don't provide the answers. Dean, on this site, recommended a while back taking 6 months to investigate. It seems like a long time but there are so many nuances, some of them very frustrating, that the time investigating is time worth spending. There is no right or wrong in the system but some systems might designed that fit your needs better.