McDonalds arrived in South Africa in the second half of the 1990's, when it was politically correct to do so, and have become ubiquitous. They are quite popular with many teenagers and other undiscerning trendoids. I am proud to say that I have never stepped inside a McDonald's in South Africa. I wouldn't do so either unless you paid me a whole heap of cash.
Other global, I mean American brands that need no local introduction include The Seattle Coffee Company, Subway and KFC.
Spur Steak ranches are the original, essential South african fast food chain, right down to the fake American name, the fake leather seats the and cheesy Wild West décor.
Steers are a lot like Spur. Spur are a bit more upmarket and they've been around longer than Steers, since the 1970s or something. Burgers, chips and other bits of greasy cow meat are a speciality, but there are also grilled mushrooms dipped in monosodium glutamate, log-cabin waffles (no, I don't know what the waffles are doing in a gosh darned African Log cabin) and other tasty unhealthy treats.
They play loud pop-rock, cater mostly to sit-down diners, but will do takeaways.
Wimpy have also been around for a while, and serve burgers and stuff. I never go there so I can't say more.
Kauai are quite new and not everywhere yet. My favourite - this is just about the only fast food that I'm willing to eat regularly (I am a vegetarian). They are Hawaiian themed with lots of fruit juices, salads and other health-conscious and vegetarian options on the menu. The smoothies are good. They cater mainly to take-aways.
Nandos were originally a South African franchise, but have since branched out into the rest of the world. Hot Portugese/Mozambique-style peri-peri chicken is their mainstay. They also do spicy rice and chips. This concept is not as surprising as it may seem, given the community of people of Portuguese extraction in Southern and south-east Africa. The name comes from a founder, Fernando.
There are several pizza delivery services, notably Butler's, St. Elmo's and Debonair's. Of these, Butler's is my choice for fast delivery, friendly service and great pizza.
Mmm, pizza! This is not American style pizza. Even the thick base has a thin crust compared to those substandard fluffy pies that you Yanks eat.
If you want to avoid the franchises, you can also get frozen yoghurt, Indian food, Fish and chips, Chinese food or Indian Food from any number of small shops. Just up from fast food, Cape Town has plenty of decent but affordable restaurants. Unless you're really poor you should try them. Italian/Mediterranean (i.e. more Pizza!) is common, but you can get others such as Thai or Sushi
Indian may be most "authentic" since the South African Indian community has been around for hundreds of years and has developed it's own dishes. Watch out for Durban Curry and bunny chow.
Or you could try the mystery meat and boerewors rolls from a person cooking it at the side of the road.
NB: Throughout South Africa and this writeup, chips means what Americans call French fries. Chips can also mean what the English call Potato crisps, but not in this node.