It's not quite that simple. Krabi is an entire province, and while there is a Krabi Town in there, you probably won't even see it. Ao Nang is the most developed beach strip, while Rai Leh is a scenic headland nearby which is not reachable by road. Ko Phi Phi, Ko Lanta, Ko Jum etc are all parts of the province. An overview:

http://wikitravel.org/en/Krabi_%28province%29

For what it's worth, when I visited Krabi a while back, we stayed at Ao Nang (which had a good selection of hotels in all price ranges) and daytripped to Rai Leh (kayaking) and Phi Phi (diving). It worked out quite nicely. ^