Koh Tao (Thai: เกาะเต่า) is the smallest of the three islands with a surface of 21 km². The island has some 1500 inhabitants and they depend on tourism, especially diving and snorkling industry. Nowadays rock and cliff climbing becomes more popular. Koh Samui and Koh Pha Ngan are the other two island and easy to visit by ferry.
There is an airport on Koh Samui and you can book your ferry tickets here on the Lomprayah website.