Groups are a collection of objects, while a symbol is a special object in Vectorworks that is designed to be used with repeatable objects. The symbol uses the concept of an instance (a symbol on the drawing) and the definition (the information used to create the symbol). When you place a similar new drawing you are placing an instance, when you edit the symbol either from the drawing or from the resource browser you are editing the definition. When you edit the definition of a symbol every instance of that symbol in the drawing will update. I will be looking at symbols at the Design Summit in Chicago.