Tibet Permits - for entering and travelling Tibet
TTB Permit (Tibet Entry Permit) and PSB Permit (Aliens' Travel Permit) are the two special permits needed for entering and travelling Tibet.
TTB Permit - Tibet Entry Permit
All foreign tourists and Taiwan tourists need to apply a Tibet Tourism Bureau (TTB) permit to get into Tibet.
TTB Permit is also called Tibet Entry Permit and it is issued by Tibet Tourism Bureau. It is necessary for entry to Lhasa or any other parts of the Tibetan Autonomous Region. When you fly from any city to Lhasa you will need to show this permit when you check in at the airport. Furthermore, whenever you buy a flight to Lhasa you need to show this permit.
TTB permits are also needed by groups traveling by Land Cruiser but this will be arranged by the travel agency organizing the trip. Your travel agency will assist you in applying for it and it normally takes 2 week days and the cost is 50 yuan/person.
Here is a picture of TTB permit.

PSB Permit - Aliens' Travel Permit
At present, some regions of Tibet are still officially closed to foreign individual tourists (See below). If you want to go further to the "unopened" areas, such as Yarlung Valley, Gyantse, Samye and the Everest region, you should have an Alien Travel Permit, which is issued by by Foreign Affairs Section of the Local Public Security Bureau.
Your travel agency will assist you in applying for it and it normally takes several hours and the cost is 50 yuan/person. Here is a picture of the PSB permit:
Tibet Unopened Areas: places of Tibet which are listed as the closed areas
Tsedang: Samye Monastery, Tomb of Tibetan King, Trandruk Monastery, Yumbulakhang
Shigatse: Sakya Monastery, Mt. Everest, Rongphu Monastery
Gyangtse: Pelkor Chode Monastery & Kubum Stupa
Ngari Region: such as Mt.Kailash, Lake Manasarovar, Tsaparang, Ali, ect.
Nyingchi Region: Such as Basum-tso, Pomi, Rawo-tso, ect.
Chamdo Region: Such as Chamdo, Riwoche, Tengchen, ect.

Tibet Travel Documents
Air Travel to Tibet
Train Travel to Tibet
High Altitude Travel Tips