The update fixes a whole number of small bugs and annoyances. A number of bugs in Spaces I heard a few complaints about have been fixed, most importantly "resolves an issue in which switching to a different space and returning back to the original space may reorder the application windows with a different active window.". Mail.app also received some love, but from the list I'm not able to tell if they fixed the issue where you cannot drag a subfolder back to root in the mailbox structure.
As always, you can either just use the Software Update tool (recommended) or download the standalone packages, which come in delta form (update from Mac OS X 10.5.2 to 10.5.3) or in combo form (update from Mac OS X 10.5, 10.5.1, or 10.5.2 to 10.5.3). These will become available from Apple's website.