Windows® XP Service Pack 3 (SP3) includes all previously released updates for the operating system. This update also includes a small number of new functionalities, which do not significantly change customers’ experience with the operating system. This white paper summarizes what is new in Windows XP SP3.
what i have been told that its similar to SP1 for XP. It's basically a roll up of all the patches and everything that has been out since SP2. There are some updates to some stuff but that hasn't been made public yet. More information about SP3 for xp will be made public more close to when its gets officially released. The download is more for those who wanna test it out for any bugs. I believe it will be released publicly sometime in june as well as SP1 for vista.