General Word About Tibet Documents needed for a trip to Tibet:
Like the trip to any other countries, it is the same that you must have the valid passport, China visa, and furthermore you must also have a Tibet (entry) permit for the trip to Tibet. And if you go further into Tibet, like going to Tsedang, Everest, and etc, you have to have some other permits or "travel allowance" as well.
There are altogether 4 kinds of documents related to Tibet travel, they are:

China Visa:

Tibet is part of China, under the Chinese Central Government's Sovereignty. Thus to travel to Tibet, you have to obtain a China Visa at first stage before all are followed up.There is no Tibet Visa or something like this. There is only China Visa or Tibet permit.If you get into Tibet via Kathmandu, you still need to apply another China Visa (this is under the Agreement between China and Nepal governments), no matter whether you already have a China Visa or not.
Tibet Entry Permit: Travel to Tibet, a Tibet Entry Permit issued by the Tibet Tourism Bureau (TTB) is needed, although you already have a China Visa. Without this permit, you can not aboard the train or plane.

Military Allowance: For going to more remote area, you need more permits. For example, if you travel overland (from mainland China) to Tibet, besides the documents stated above, you still need the Military Allowance.