There is plenty of information about what Revit Structure does and how it is used at:
http://www.autodesk.com/revitstructure
Particularly look at the links on the left hand side of the page:
"Frequently Asked Questions"
"Product Brochure" (a pdf file)
"Q&A" (another pdf file)
Your reseller would probably be glad to talk to you about the differences between Revit and AutoCAD as well.
Joe