Note: This guide is for the 2007 and earlier SharePoint Products. Forums for the 2010 SharePoint products can be found here:
Very roughly General is for Pre-Sales and other kinds of General questions. This therefore includes things like which SharePoint versions there are; Licensing; asking for general suggestions on which approach
to take; and questions on "is there a product for X?". It also includes questions on Application Templates because these aren't part of the installed product and thus don't fit in for instance Admin.
What it doesn't include are questions on a specific problem - "I can't access the SP site"; "I'm getting error message Y"; Where is Z in Central Admin?; stsadm questions etc. etc. All specific questions that aren't about anything covered in forums like Search;
Workflow etc. and that are connected broadly speaking with Administration, go to the Setup..Admin forum.
These days (2010) I'm moving all such questions that come to General to the Admin forum.