Ko Phi-Phi is actually two islands about 40km from Krabi: Ko Phi Phi Leh and Ko Phi Phi Don. Both are part of Hat Noppharat Thara Ko Phi-Phi National Marine Park, though only parts of Ko Phi Phi Don are actually under the administration of the Park Division of the Royal Forest Department.
After Phuket, this is the most popular tourist destination along the Andaman coast. During the high season from December to March, multitudes of travellers, descend on the island and snatch up every available room and bungalow. The island still retains some of its original beauty, but to truly appreciate it you'll have to take a long hike to escape the crowds.
Ko Phi-Phi Leh is almost all sheer cliffs, with a few caves and a sea lake formed by a cleft between two cliffs that allows water to enter into a bowl-shaped canyon. The so-called Viking Cave (Tham Phaya Naak) contains prehistoric paintings of stylised human and animal figures alongside later paintings of ships no more that 100 years old.
Ko Phi-Phi Don looks like two islands connected by the bridge of land only a few hundred metres long. This 'bridge' forms the beautiful double bays of Ao Ton Sai and Ao Lo Dalam, both of which have long, white-sand beaches. The mostly uninhabited western half of the island is called Ko Nok (Outer Island), and the larger eastern half is Ko Nai (Inner Island). The the north end of the Ko Nai is Laem Thong, where the island's chao naam populations lives. The number of chao naam here varies from time to time, but there are generally about 100. Ko Phi-Phi's topography boasts scenic hills, awesome cliffes, long beaches, emerald waters and remarkable bird and sea life.