As far as I can tell, taxis are local people who own a car and wish to make money from it. Perhaps they have special licences but it’s hard to tell. As you step out into a public place (like exiting an airport, crossing the border or getting off a mini-bus), you are immediately swamped with taxi drivers. The first one to grab your bag wins the fair, apparently, and so they all battle over the luggage.
Like the mini-buses, taxis usually only leave when they are full (hence all the guide books refer to “shared taxis” – there’s really no choice).
Our mini-bus from Chipata to Katete, a 16-seat van, waited 2 hours before it left the bus station. For the last hour all the seats were full, but people kept coming along needing a ride, so the driver kept cramming passengers in. By the time we left there were 27 passengers on board, including 2 screaming babies. A colourful ride indeed.
EDIT: We hired a driver to take us to Chipata, a 90km trip to from Katete. On the way we were stopped by police and he was told of for “pirating”, or acting as a taxi illegally – they wanted him to paint his car blue to mark it as a real taxi. I’ve never seen a marked taxi, this is the first I’d heard of it. He had to pay a K54,000 fine (about $15) which I think was more of a bribe, before we were able to move on.