CNC Control Software is one of the fundamental things you will need to do CNC. The control software is what interprets the G-Code line by line and sends off commands to your machine controller.
Probably the standard in the hobby market is Mach. There are different versions: Mach1, Mach2 and Mach3. Mach is created by a company called Artsoft and has an excellent reputation in the marketplace. The software is easy to use and very powerful. The company also provides great service and is always working on the next generation of product.
I have used a couple other control software packages as well like CNC Pro, TurboCNC and Master. They seem a bit outdated compared to Mach, but serve their purpose and work well.