Ironically known by many as the "Black Taxi", these minibus taxis aren't black at all. The reference to "black" is because for the most part they're black-owned and mostly transport our black population to and from their respective places of work.
Generally one finds taxi ranks near large shopping malls, train stations and in certain suburbs. These taxis drive predefined routes and pick passengers up and drop them off along this route. Never having used such a taxi, I'm not completely sure of how the cost of the trip is determined - I don't believe there's a flag (since passengers are getting on and off all along the route) so I imagine it's up to the discretion of the driver or his assistant.
Minibus taxi drivers are notorious for not obeying traffic laws. They're almost a law unto themselves, passing on the yellow line, pushing their way between cars to the front of the queue, and driving faster than perhaps they should be. Normal cars play the chicken game with taxis, to see who will back down first and generally unless the car clearly has the upper-hand, the taxi wins.