It goes by a few different names. I think everyone has a different idea.

Computer Aided Machining
Computer Aided Manufacturing
Computing Aided Maneuvers

With all that said, you can think of it as motion control. Controlling motion of a machine with a computer, which is running a program. This program is the G-Code you have heard about. It tells the machine when and where to move.
Generally, CAM is a type of stand-alone software that you will have to buy. When you are using CAM, you will need to know the type of CNC machine you will be using, the tooling you will be using, and the feeds and speeds of the machining process you will utilize. You will input these variables into your CAM software and it will create a program. You will then use this program to control your machine's motion.

The output of CAM software is G-Code. G-Codes are simple text commands used to instruct your CNC machine. Usually you'll need to know some basics about G-Code, but generally you don't have to be an expert at it. If you really like CNC, you will naturally start picking up how to read G-Code. If you get good, you can even hardcode some of your G-Code.

This Computer Aided methodology is applied in different manufacturing areas in different ways. In CNC, manufacturing with a CAM system is used to simplify the machining process. In most cases, the CAM system will work with a CAD design made in a 3D environment. The CNC programmer will just specify the machining operations and the CAM system will create the CNC program.

Ivan is active in CNC Programming and G Code. CAD CAM is one of his expertise.