Building Information Modeling vs. Computer-Aided Design
BIM is Building Information Modeling. It is an integrated workflow built on coordinated, reliable information about a project from design through construction and into operation.
CAD is Computer-Aided Design. CAD is simply the use of computer systems to assist with design. While BIM tools such as Revit are technically CAD programs, the term "CAD" is used more for drafting programs. In terms of building design, CAD is essentially using a drafting tool, such as AutoCAD, to create lines and arcs to represent a building design.
Basically, when using CAD for building design, you focus on creating drawings. When using BIM, you focus on creating a building model and then the drawings can be generated from the model. This saves time because you really do not have to draw anything twice. With CAD, you typically draw floor plans and then sections. But with BIM, you create the elements in one view, and they appear in all views in which their visibility is turned on.
Please note that Revit is not BIM. Although Revit was designed for BIM, it does not accomplish every aspect of BIM. At the same time, when using Revit to create a building model, users can quickly export the model to other programs. This is useful for producing higher-quality renderings, performing analyses, coordinating with other disciplines, fabricating parts, and much more. With CAD, many of these additional options aren't available which means the interoperability with BIM is much greater.