They are interchangeable. The difference is: Begin is formal Start is informal

Yes you say, 'We can start whenever you are ready.' As Craig says they are mostly interchangeable. However there is a nuance in some cases with time between these two verbs You can start a car and then begin driving. Start has the sense of an action happening immediately. You can never begin a car. You can start the machine you can never say you begin the machine.

Actually, is 'between' not 'about' =)