Apart from the usual round of fixes this update includes lots of new functionality for the newest two members of the Office family: OneNote and InfoPath.
To get SP1 installed go to the Office Update web site click check for Updates.
New features in OneNote include better integration with the other Office apps and shared real-time note taking! A complete list of all the new features is available here : http://www.microsoft.com/office/onenote/prodinfo/sp1/overview.mspx
New features in InfoPath include support for custom ActiveX controls, inking support, better digital signature support, easier validation and support for more complex XML structures. If you are a developer using InfoPath today you must go get this update as it vastly improves developer productivity. A full list of all the improvements is available here: http://www.microsoft.com/Office/infopath/prodinfo/sp1.mspx
UPDATE: See here for steps to install SP1 on a machine that has InfoPath SP1 Preview installed on it.