The 2007 Microsoft Office suite Service Pack 1 delivers important customer-requested stability and performance improvements, while incorporating further enhancements to user security. This service pack also includes all of the updates released for the 2007 Office suite prior to December of 2007. You can get a more complete description of SP1, including a list of issues that were fixed, in the Microsoft Knowledge Base article 936982: Description of the 2007 Microsoft Office suite Service Pack 1.
Service Pack 1 will install even if a publicly available update has already been installed on your computer.
SP1 also includes stability improvements developed as a result of user input from the Microsoft Online Crash Analysis and from Microsoft Product Support feedback.
See whitepaper for full list of improvements: http://office.microsoft.com/search/redir.aspx?AssetID=AM102512381033