OCModShop's own photography journalist has written her premiere article about the differences between amateur cameras and professional gear.
Here's a snip:
The strictest distinction between an amateur and a professional camera is the lens type. SLR (single lens reflex) cameras have the ability to swap lenses. Point and shoot cameras have a fixed lens that cannot be removed. All point and shoot cameras are amateur cameras. However, not all SLRs are professional cameras. Professional SLRs all have approximately the same size lens fitting.
If you buy a camera with very small lenses you can be sure that this is a nonprofessional SLR. Many cameras today also balance on the line between amateur and professional cameras. This is where the debate becomes more a matter of opinion then a technical one. Read on over here to learn more..