All cichlid species come from the tropics. However, the African cichlids belong to a special sub-group of tropical fish. African cichlids have very specific water requirements and an aggressive temperament that make it hard to find tank mates. However, finding suitable tank mates is not totally impossible.
When aquarium hobbyist say "African cichlids," the term is understood to refer to cichlids from the African Great Lakes in the Rift Valley. The majority of Africa's cichlids come from these lakes, and the handful of other species in the aquarium hobby are usually called "west African cichlids" or "African river cichlids." In fact, more cichlids come from Africa's Great Lakes than any other region of the world, including the rest of Africa, the entire continent of South America and all of Central America.
African cichlids have unusual water preferences. These fish prefer very hard and alkaline water. In fact, African cichlids come from water harder and more alkaline than seawater. This water got this way by dissolving calcium and other minerals from the surrounding bedrock. This water chemistry makes it difficult to find tank mates. Fish get stressed and more prone to disease when kept outside of their preferred pH range. For example, South American cichlids prefer, soft acidic water and wouldn't last long in this kind of water. Other than African cichlids, very few fish can tolerate such extreme water chemistry.
Above and beyond their water chemistry, African cichlids' demeanor bar most fish from sharing a tank with them. This group of fish has a reputation for aggression and will often fight with other fish. In particular, cichlids will often attack other cichlids, especially cichlids from other areas. Some African cichlids, like peacock cichlids, are less aggressive than the rest, so there are African cichlids that may tolerate tank mates better. Always research the specific fish you want before trying to keep it with something else.
What Does Work
A few species of catfish come from the African Great Lakes and can share a fish tank with African cichlids. The Malawian upside-down catfish (Synodontis njassa) comes from Lake Malawian, a lake home to hundreds of species of African cichlid, and this catfish is robust enough to live with them. Additionally, some midwater schooling fish like Australian rainbow fish and Congo tetras can live with African cichlids. These fish can tolerate the same water conditions and can usually run away fast enough to avoid cichlids' wrath. However, some African cichlids specialize in eating smaller fish and cannot share a tank with these bite-sized tank mates.