The cities with international airports are Tunis-Carthage, Monastir and Jerba. The national airline is Tunis Air flies to many cities in Europe, North Africa, and the Middle East, but does not service the America, Asia, or Australia.
There are only 4 airports offering domestic travel in Tunisia, thus if you are planning on seeing a few places while you are there, you may want to plan on using buses or trains. The buses run by the national company are in good condition, air-conditioned, and fast, and they have very comprehensive routes. There are also regional bus companies (slower and not air-conditioned). In the summer buses operate at night to avoid the high heat. The trains are not as comprehensive, but are modern and run on schedule. Another option is the louages, which are 5-seater long-distance taxis. They only depart when full (not a problem) and cost only slightly more than buses, but are more direct.