The way these things work is pretty simple. If you're at the taxi terminal, you find the appropriate rank and get in a taxi going to your destination, and pay the conductor. Then you wait for the taxi to fill up to the satisfaction of the driver, and you take off.
If, as is more likely in town, you're somewhere along the route, you wait until a minibus taxi goes by. You'll know even before you see it, because the driver will be tapping the horn as he sees a cluster of people waiting for a bus, and his helper will shout out the destination of the minibus taxi. Wave it down. The helper will slide the door open, and find you a place to sit. There is *always* room for one more passenger.
You can get pretty much anywhere in Southern Africa by minibus taxi, but I made it a rule only to take them in town, mostly for safety reasons, but also for comfort. They are easily the cheapest way to get around, but you get what you pay for. Minibus taxis are renowned for their lack of maintenance, their insane drivers, and their sardine-can conditions.
In Maputo, I rode in one which carried 21, including the driver and his helper/conductor. This was somewhat tight, as the helper had to sit on the window ledge with his head and body outside the minibus, but I've seen worse, and it simply made it easier for him to shout out the destination.
An Australian couple I met in Maputo had taken a minibus taxi on a long-distance journey to Cape Town and nearly asked to get off in the middle of nowhere after several hours of screaming down roads at 160km/h, overtaking on blind corners, and being crammed in tight. They swore never to take one again, at least not for a long journey.
Minibus taxi ranks are also great places to get robbed; one German guy had just gotten off one at the terminal. He put down his pack for a second, and it was instantly whisked away, along with his passport. Oops.
Having said that, they seemed to be fine for local trips, where you can't go fast anyhow, and being crammed in for a ten or fifteen minute ride just isn't so bad.
There are minor local variations: in Maputo, they're called chapas, you don't pay your 20 cent fare until you're ready to get off, as the act of handing the conductor your fare is a signal that you want to stop. In Durban, the fare is 28 cents, anywhere in town, and they even carry signs in the window naming points on their fixed route. In Gaborone, which seems to be farthest along the line in systematization, there are destinations and route numbers painted on the front of the minibus, and even advertisements on the side of the bus.
It's no surprise that Gaborone is in Botswana, one of the richest countries in Africa, and Maputo is in Mozambique, one of the poorest.
These minibus taxis actually remind me of the public light bus and maxicab system in Hong Kong, and it's possible they'll evolve into something similar. At the beginning, Hong Kong had fleets of these unofficial minibuses, unregulated and chaotic, much like those here in Southern Africa. Over time, they became more and more orderly and regulated.
Nowadays they're even listed in the official Hong Kong guides to public transit, along with their routes and times. Some of them have those irritating multimedia screens onboard. And latest word is that a lot of them now accept Octopus, the Hong Kong proxcard fare system. That's about as far from the low-tech chaos that exists in Southern Africa as I can imagine. But they started out the same way, and there are still a zillion and one different maxicab/public light bus companies operating, all over Hong Kong, Kowloon, and the New Territories.
A minibus system like this pops up (as it has in New York) because people need to get around and whatever existing ways to get around aren't serving them. Hong Kong evidently decided that the best thing to do is to licence and organize the system, instead of marginalizing it. (I'm not sure what New York is doing with its "dollar van" services.) In South Africa, much of the public transit infrastructure has suffered for decades as the rich abandoned it for private cars, but the need for it hasn't gone away, as poor people still need to get around.
Minibus taxis are relatively cheap, compared to full-size buses, and they don't require drivers with specialized licences. (As a matter of fact, in Southern Africa, it's an open question as to whether your driver even *has* a licence.) So the cost of entering the business is low.
It's possible that as African countries develop, the minibus taxi system will become more and more organized. On the other hand, I don't know that Hong Kong had as much of a problem with the other reason why minibus taxis are considered unsafe by much of the white population: minibus taxi wars.
Apparently one of the ways minibus taxi companies compete is by discouraging riders from taking the minibus taxis of their competitors is to shoot at the competition, and kill the driver, conductor, and passengers. Johnny, our driver on the backpacker bus I took through the Transkei, pointed out a place where such an ambush took place. "They hid back here," indicating a small ridge along the side of the road, "and when the minibus taxi came along they opened up on it. AK's, 9mm's, everything. Killed everyone on board." A truck driver was unlucky enough to be following. "He was driving home with an empty load," said Johnny, "but they didn't want any witnesses, so they shot him too." Johnny says he came upon the scene half an hour later, when the road was blocked by the police investigation. "Half an hour earlier, and *I'd* have been that truck driver."