Staad is an analysis program with modelling capabilities built in.
Revit is a modeling program with documentation (paper drawing creation) features built in.
Revit Structure also links to analysis programs- you can export your Revit model to Staad, for example, and analyze it, then bring Ute changes ( based on analysis results) back into Revit.
More info:
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)
Joe
