AutoCAD and Civil 3D

What is the difference/relationship between AutoCAD and Civil 3d?

AutoCAD has been around for ages. It's the base platform for Computer Aided Design (CAD). AutoCAD is an excellent way to show information in a 2-D format. You can draw in 3-D but in the Transportation Consulting world it's hardly used at this moment. 

Civil 3d is an 'add on' to AutoCAD much like how Land Desktop Developer is (or was).  Civil 3d is a fantastic analytical tool to help designers and engineers make critical design decisions. It also minimizes the guesswork and interpolation by producing accurate results quickly and efficiently in REAL TIME.

Let's put it this way. AutoCAD is your Motor and Civil 3d is the Turbo Charger which you attach to the motor. There are plenty of field specific add-ons offered by Autodesk but as a road or land developer, 99% of your needs will be answered through Civil 3d.