Revit Architecture and Revit Strkucture are architectural design and production tools with a high focus on inter-coordination of all the parts and pieces. So, to be clear, Revit can do 3D and 2D models, completely fully coordinated. Anyway, most of people also use AutoCad early in the conceptual design phase and then import the resulting 2D sketches or 3D models into Revit as the basis for further design.
To learn how Revit & Autocad can work together read more on the link