Tummy is very informal, and is often used by children. Belly is also rather informal. Stomach and abdomen are more formal. Stomach typically refers to the internal organ where digestion occurs, while abdomen refers to the part of the body between the chest and the pelvis which contains the stomach and other organs.

.Another common usage: we say someone "has a belly" if they are overweight

I agree with Susan, I'd only add abdomen (at least in my region) more often refers to the stomach muscles.