What is the difference between civil and architectural drawing?

The main difference is industry the CAD is aimed at. But really if you break it all down into its simplest form there is no difference. CAD is CAD. Its all just lines and circles.

But if we look at the actual software we find there are specific softwares related to specific industries for instance AutoCAD Revit Suite (fantastic btw) caters for 1)Architectural 2)Structural 3)Mechanical, Electrical and Piping. Its a program split into 3 but interconnected (check it out) but the use of the program and functions are all the same.

However if you really mean mechanical as in part moDELLing then there is a huge difference.

AutoCAD Revit is an area of CAD called Building Information MoDELLing (BIM) this is where everything is basically library based in its simplest form. A huge parametric library of components is added to lines and points you've drawn on a plan. The properties of these components are then altered through a specific list of possible configurations and editable variables.

Solidworks on the other hand is another form of CAD which is the one I think you mean by mechanical. But really you can do any type of moDELLing with it if you know what your doing. Solidworks is a dimension based part moDELLer. It can create solid parts, surface parts, sheet metal parts and much Much MUCH more. In this type of CAD you take an idea or a sketch (more commonly a doodle) and create a custom part from scratch. There are libraries involved in there too but for the most part its usually from scratch.

Both types have their own specific simulation systems. And their are other abilities each of these CAD types can do but I just wanted to scratch the surface or else this answer would be a proper report!