First of all, your Use Year has nothing to do with when you can make a reservation or your dates of travel. You can book a reservation at your home resort 11 months prior to your check-out date regardless of your Use Year. The only thing that matters is that you have enough points in the same Use Year as your dates of travel. If you didn't have enough points in that Use Year, you could bank points from the previous UY and/or borrow points from the next UY.
If you have a December Use Year (Dec 1 - Nov 30) then your points work as follows:
Your 2005 UY points are valid for stays from Dec 1, 2005 - Nov 30, 2006
Your 2006 UY points are valid for stays from Dec 1, 2006 - Nov 30, 2007
Your 2007 UY points are valid for stays from Dec 1, 2007 - Nov 30, 2008
etc.
Your Use Year also dictates your banking deadlines. With a December UY:
You can bank 100% of your points by May 31st
You can bank 50% of your points by Aug 31st
You can bank 25% of your points by Sep 30th
You cannot bank any points between Oct 1st and Nov 30th (end of UY)
Banking is cumulative. For example, if you already banked 50% or more of your points by May 31st, you cannot bank any more points after that date. If you had banked 40% of your points by May 31st, you could bank another 10% between May 31st and Aug 31st.
Where Use Year really becomes important is if you have fairly regular travel dates and if you think you might ever have to cancel a trip. If you have a Dec UY and book a trip for Jan '07 and then have to cancel it, you will have 11 more months to use those points for something else before they expire (on Nov 30, 2007). You are also well within your 100% banking deadline and could bank those points into the next UY. However if you normally go in January and had a February UY, then canceling a January trip is a huge problem -- your points would expire on Jan 31st so you would have no time to use them for another trip and you are past all banking deadlines.
One more thing. If you are purchasing from Disney, you get current Use Year points with your purchase. So if you purchased a Use Year Month where you are still in your 2005 UY (Aug, Sep, Oct, Dec) then Disney starts your contract with 2005 UY points. If you purchased an earlier Use Year Month (Feb, Mar, Apr, Jun) you are in your 2006 UY, so Disney starts you out with 2006 UY points. So by purchasing a later Use Year, you get an extra year's points allocation with your purchase.