Traveling in Tibet, you need not only China Visa but also Tibet permit. There are 3 kinds of permits, Tibet Entry Permit(TTB permit), Tibet Travel Permit(PSB permit) and Military Permit.
Tibet Entry Permit
It is also called TTB permit, issued by Tibet Tourism Bureau. You need it to help you enter Tibet. When you fly or take a train from any city to Lhasa you will need to show this permit when you check in at. Furthermore, whenever you buy a flight or train ticket 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.