Many people often mistook the Tibet permit as a visa. However, Tibet is a part of China, not an independent state. So, if you want to travel to Tibet, you must first apply for a China Visa to the nearest Chinese Embassy.
Generally speaking, Tibet permit can be divided into three categories:

Tibet Entry Permit
1. Tibet Entry Permit: issued by Tibet Tourism Bureau. Needed if you want to travel to Tibet.
Please prepare the following documents ready before asking your travel agent to apply permit:
A copy of your Passport;
A copy of your China visa (tourist visa);
Full names, gender, Date of Birth, Passport number, nationality, occupation.

Aliens' Travel Permit
2.Alien’s Travel Permit: issued by Public Security Bureau. Needed if you want to travel to some unopened areas in Tibet.

Military Allowance
3.Military Allowance: issued by military office in Lhasa. Needed if you want to travel to some remote and military sensitive areas.