Tunisian shared taxis; the name comes from the French word louer, which for all you non-Francophones means "to rent". Louages are easily-identifiable white minivans or station wagons marked with red or blue stripes indicating their range of service. (They also have signs on top, like many taxis, but these signs are generally in Arabic, which I can't speak, let alone read, so they're no help to me.) Blue-striped louages are local, limited to a particular zone; the red-striped ones are reserved for longer trips that go from one zone to another. Louages generally seat between five and eight passengers, depending on the size of the car, and drivers are eager to fill up as they earn a fixed, per-person fare depending on the city of origin and destination. If you're in a hurry, you can offer to pay the driver for the empty seats in your louage, but otherwise be prepared to hang around the louage station waiting for the car to fill to at least more than half its capacity. Your driver may also stop to pick up other passengers along the way, which may further delay your trip, so it's best to have an idea of how long a drive you're expecting and budget a lot of extra time for a trip in a louage.
To get a ride in a louage , either go to the local station in the city you're in or walk along the road between the station and your destination and try to flag down a correctly-striped car headed your way (towards the station is okay, as the driver will turn around as soon as he picks up enough passengers). The red and blue louages sometimes have different stations a short walk from each other, but there are generally regular yellow taxis at both stations, so you can take one of those for the short distance between the two if need be. At the station, just wander around asking drivers the name of your destination city, and someone will direct you to a car headed your way. Once you get in the louage, confirm your destination and if possible the price. It's best to 1) know the price in advance; and 2) have exact change in case your driver's the type who tries to make a few extra dinar by playing dumb or just plain rude to tourists instead of making correct change. Some louages have price lists (unfortunately for me, I've only ever seen them in Arabic) but these are the exception rather than the rule.
Your ride will take you to the louage station in your destination city, or any point along the way there, as long as you pay before getting out. I like riding to the louage station, just to know where it is, plus it's easy to catch yellow cabs or a different color of louage there. There must be some kind of arrangement between the taxis and louage drivers, because they don't seem to compete much; as mentioned before, it's possible (and common) to take a taxi the short distance to the louage station before getting into a different car for a longer ride. Maybe the taxis make enough money off short local rides and larger fares from tourists who don't know about taking louages long-distance.