i think that rape involves intercourse / penetration when the person has not or is not in a position to give consent (thus that covers cases where the person actively doesn't consent and also cases where children are too young to be able to give consent) whereas sexual abuse is a bit broader (touching and stuff). but... thats just my hunch.

ian hacking has a lot of interesting things to say about 'abuse' and about how our concept has changed over time... really very interesting...