The CNC controller takes the instructions (or code), interprets it and executes the commands. It does nothing more than what it is told (programmed) to do. Today, controllers are complex and can also perform other tasks. For example, you can program the controller to do a dry run, etc.
Hopefully the above has given you a better understanding of how a CNC machine works. For further reading on buying a CNC, see The Stone Shop column that appeared in Surface Fabrication magazine's May 2007 issue, "Is A CNC In Your Future?"
About the author: Fred Hueston is a worldwide expert on stone installation, failures, fabrication and restoration. He is the founder of The National Training Center for Stone & Masonry Trades (ntc-stone.com) and Stone University (stoneuniversity.org). He can be reached at Fhueston@aol.com.