CG is a g-code circle. The shopbot help provides a good explanation, but you can also look at g-code tutorials as well. The only real difference is that g-code separates clockwise and counter clockwise circles into two different commands where shopbot adds a direction variable. I usually find this guy pretty helpful, although I have not read this particular article.
http://www.cnccookbook.com/CCCNCGCodeArcsG02G03.htm