A few days ago we released the reference documentation for the TFS Build workflow activities.  There are a lot of them and they are as multi-faceted as a swiss army knife - so having the reference documentation is darned handy.  You can check out the docs here: Team Foundation Build Activities.

You can also read the author's blog post about them to get a little more context on how to navigate the docs: http://blogs.msdn.com/b/andy-lewis/archive/2010/12/10/team-foundation-build-activity-reference.aspx