Overall, Intercape Mainliner is the best option for Western Cape destinations. Other operators include Greyhound, Translux/City-to-City (for less serviced destinations like Umtata), and Baz Bus. Common routes from Cape Town include Johannesburg and Tshwane, Springbok, Windhoek, George, Port Elizabeth, and Durban. All the main bus companies operate from the bus terminal alongside the central train station on Adderley Street, and most have their offices there.
Within Cape Town, Golden Arrow runs an extensive network of routes from the main Golden Acre terminal on the Grand Parade (Castle Street side). These city buses are by far the cheapest form of transportation (much to the frustration of the minibus taxi operators). You'll get to most destinations for R5-R10, and you can save by buying 10-ride clip cards. The bus runs from the central train station to the Waterfront (R4.10) and to Kirstenbosch (R6.90), and there's also service departing from the Cape Town Tourism Information Office in the city center.
The service has a timetable, but buses often run late and you'll need a certain level of knowledge regarding its operation. Bus shelters and lamppost markers indicate stops. Route maps are not available in leaflet form, but they are displayed at all major depots. Alternatively, phone the Golden Arrow hotline, or ask people at a bus stop for info on which ones go your way.
For short trips locals generally use minibus taxis, which waste no time getting you to your destination and, for the modest fare of R5-R20, provide you with some local atmosphere. You can hop on and off the combis (minivans) quite easily; small gatherings on the roadside usually indicate a stop. However, don't expect to leave the starting point until the taxi is full, which can slow you down outside of peak hours or away from busy routes. The main minibus stop in the city center is above the train station on Adderley Street, but you can flag combis down just about anywhere. Most taxis are sound, but watch out for those held together with tape and wire (literally). There are no route maps for minibus taxis; ask the drivers where they're going. If you don't have the exact change, you may have to wait until the guy taking the fares gets your change. Overcharging is not common, but it's best to discreetly ask other passengers what the fare should be. Taxis are crowded, so watch out for pickpockets.