Phi phi island is thailand's island-superstar. It's been in the movies. It's the topic of conversation for travelers all over thailand. For some, it's the only reason to touchdown in phuket. See more details: http://www.andamanferryservice.com/phuket.php
Ao Nang (sometimes known as "Ao Phra Nang", a name the Tourist Authority of Thailand devised some years back, but which never really caught on) is the most "westernized" beach in Krabi. Originally a backpacker hotspot, it is now moving rapidly towards the package tourist crowd as the airport brings in more charter flights with each season.