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.
Taken from the Microsoft website itself
Another interesting info from the pdf :
Microsoft is not adding significant functionality from newer versions of Windows, such as Windows Vista, to Windows XP through XP SP3. For instance, Windows XP SP3 does not include Windows Internet Explorer 7, although Windows XP SP3 does include updates to both Internet Explorer 6 and Internet Explorer 7, and it will update whichever version is installed on the computer.