What is the difference between AutoCAD MEP and AutoCAD?
Also, can AutoCAD MEP 2014 be installed on a machine with AutoCAD 2014 already on it?
AutoCAD MEP is AutoCAD plus tools for MEP design. It includes tools for HVAC, Plumbing, Piping and Electrical as well as Schematics. It is object based so it actually has objects that are ducts, pipes, circuits, fittings, etc and not just lines, blocks, etc. You can get an overview of this product here: http://www.autodesk.com/products/autodesk-autocad-mep/overview
It can be installed on the same machine as AutoCAD but you really don't need plain AutoCAD if you have AutoCAD MEP installed. It can be run as plain AutoCAD if you want to run it that way for whatever reason.