G-code

[DonRotolo]

It also depends on your CNC machine: A CNC router (generally) needs only 2.5D, while a Mill with a 4th axis would want more dimensions. The programs that generate G-Code differ wildly in price, and often have different capabilities.

HSMWorks is free, and works with both SolidWorks and Inventor.
A program called Cut2D is a good choice for 2.5D, but costs $150. The same company (Vectrics) makes other programs that are all good.
MasterCam is the gold standard IMHO, but its price is high.

Note: You can also write G-Code by hand! Like knowing how to deal with a dot prompt (Linux) or > prompt (DOS), it can be helpful to know this. For complex shapes you probably would go with automatic code generation, but you can make simple stuff by hand, as well as understand the generated code.