Although there are lots of differences between these systems (notably one being a product and the other just being a framework), there remains quite a lot of confusion for some people in trying to choose which to use. I came across a couple of good articles with information to help you make a decision between Biztalk and WF for certain projects where it may not be crystal clear.
1. David Chappells excellent Introduction to Windows Workflow Foundation at : http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnlong/html/WFIntro.asp
2. Irena Kennedys post on Biztalk or Workflow at :http://softlogger.com/4314/BizTalk/SYSK-216–BizTalk-Orchestration-or-Windows-Workflow-Foundation.aspx
Hope you find these articles useful if you are at such a decision point.
(If anyone knows how to make firefox retain the hyperlink in the rich text editor or IE7 to display the rich text box instead of an ordinary text box, then please email me. Otherwise my only option is to go back to IE6).