Hi M K,
the others have covered it well. I assume you are contemplating to get into this business.
One thing I would advise is not to under-charge when you first start out. It is often tempting to do that because that certainly is a way to get clients. But you need to get up to the rates that the market will bear assuming similar credentials, and it can be difficult to start charging more or, worse, finding yourself in a position where you charge different rates to different people.