Topics about team sites, project tracking, document collaboration, extranets, etc.
Topics about improving or expanding the worldwide SharePoint community through blogs, forums, social networking, conferences, user groups, code camps, etc.
General (typically presales oriented) questions and discussions.
Topics about Search capabilities in SharePoint.
Topics about installation/setup and upgrade/migration as well as administration and operation.
SharePoint - Social Computing
Topics about MySites, blogs, wikis, tagging, social networking, knowledge search and brokering, communities, etc.
The complete list of SharePoint forum categories on TechNet is here.