There are two documents required for foreign tourists who want to travel in Tibet. One is the Chinese Visa, which you can apply for in Chinese Embassy in your place. Another is the Alien's Travel Permit issued by Tibet Tourism Bureau.

Non-Chinese passport holders (including those of Taiwan, Hong Kong SAR, Macau SAR and other countries) must have an Alien's Travel Permit as well as a valid passport and a visa (Travelers from countries having visa exemption agreement with Hong Kong do not need a visa) to visit Tibet. Visa can be obtained from the local China consulate in your country. Since the government encourages group tours to Tibet the permit is issued only to tour groups traveling with a Chinese tour operator. Your travel agency will represent you to facilitate the application process.

Do the paperwork before you go to Tibet
There are two kinds of permits (Tibet visa) for traveling in Tibet: the entry permit (TTB permit) and the Alien's travel permit (PSB permit).

TTB Permit - Tibet Entry Permit (Tibet visa)
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.

PSB Permit - Alien's Travel Permit
If you want to go further to the "unopened" areas, you need an Alien's travel permit issued by PSB of Tibet. It is called "Alien's Travel Permit" or "PSB permit". Here is a picture of PSB permit :

Important Issues about Tibet Travel Permit
1. All kinds of people can get Tibet permit through a travel agency except diplomats, journalists, and government officials who should travel to Tibet under the arrangement by the Foreign Affairs Office of Tibet Government.

2. After having a permit, a travel agency could buy you the air tickets, and you can pass the check-in at the airport and the check point reroute with the permit.

3. Only the guide could hold the permit after you establish yourself on Tibet land. You are not allowed to bring with it and travel to anywhere you want because except Lhasa you need another permit named as "Aliens’ Permit". Otherwise if you are stopped by the police you will be sent out of Tibet or have trouble politically and economically.