There are two categories of long-distance buses in Tunisia
Both National and regional buses share bus stations all across the country. These stations are referred to as gare routiere.
National buses are operated by the Societe Nationale du Transport Rural et Interurbain ( SNTI). SNTI operates daily buses from Tunis to almost all towns across Tunisia. You can get all bus schedules and pay online via their website. I highly recommend using these buses for long-distance trips since they are inexpensive and adhere to timetables.
Bus frequencies differ for different routes.
These buses operate around their area, most regional bus service providers don't have websites and the only way to know their schedules and prices is by going to make inquiries at the bus stations. Advance reservations are difficult since most regional bus providers open their booking window approximately 1 hour before departure.
Local commuter buses are only available in major towns such as Hammamet, Tunis, Sfax, and Sousse.