CAD (Computer Aided Drafting) is, in general, a tool for computer based drafting. Screw the drafting boards and drafting pens and T-Squares! CAD is where it's at.

AutoCAD is just one particular breed, and in most industries, the most popular. Specifically, AutoCAD is a program created by AutoDesk (the makers of 3D Studio). It can handle everything from 2D drafting to 3D drafting. It can import/export to many popular 3d rendering programs such as 3D Studio Max/Vis, Form-Z, and (from what I have heard) Bryce 3D.

This program isn't the only of it's kind. It has competition: Microstation and QuickCAD are the biggest competitors. There are several others, including some Linux versions, but none compare to AutoCAD.