Koh Phangan is an island in the southern part of Thailand. It's quite quick and easy to reach. It's just thirty minutes away from the Koh Samui Island and three and a half hours away from Surat Thani province by boat or cruise. As there are no airports here, they rely on ferries coming from the mainland to travel/
Koh Phangan is very famous for its full-moon parties that take place every month sometimes twice a month. The exuberant all-night celebrations coincide with the lunar calendar. The most revelry is done on Sunrise Beach in Haad Rin. The beach bungalows here attract backpacker crowds. Most of the elderly crowd skip this island because of the full moon party, so it's a perfect place to enjoy for the young.
There are quieter beaches towards the northern coast including Hat Thian and Hat Khuat. Other than the parties, it's quite a beautiful island with a lot of white sand beaches. It's filled with lush green jungles and waterfalls too. Swimming here is not recommendable as this is a hot spot for jellyfish. If you're really into swimming and diving then Sailrock is a splendid place, the place you'd want to go.
A heads up if you hire a moped or bike to ride, traffic rules don't apply here, people ride a little rashly, so you should ride defensively. Other than this it's a perfect place and if you're in Thailand and are an adventure freak you don't want to be missing out on Koh Phangan. Also, come try out the Koh Phangan hostels for a humble stay.