The 2007 Microsoft Office Suite Service Pack 2 (SP2) provides customers with the latest updates to the 2007 Office suite (the products that are affected by this update are listed below). This download includes two types of fixes:

Previously unreleased fixes that were made specifically for this service pack.
In addition to general product fixes, this includes improvements in stability, performance, and security.
You can find out more information in Knowledge Base Article 953195, where product-specific changes are described.
All of the Public Updates, Security Updates, Cumulative Updates, and Hotfixes released through February 2009.

Before installing this service pack, you are strongly encouraged to read 953195, which describes some big improvements introduced by SP2, and also calls out some important information that you should be aware of before installing.