There are more than 2,000 species in 18 families within this large and diverse order of freshwater fish. They live in lakes and rivers throughout most of South and Central America, with at least 209 species also found in Africa. This order includes familiar fish such as piranhas, tetras, and tigerfish.

Characiformes come in many shapes and sizes. Some are small and colorful, like the tetras often kept in home aquariums, while others—like piranhas—are known for their sharp teeth and powerful jaws. Most species have well-developed teeth, which they use to eat a wide variety of foods, including plants, insects, and other fish.

Many fish in this order have a small extra fin on their back called an adipose fin, located between the dorsal fin and the tail. Not all species have this feature, but many do.

Most Characiformes are small and brightly colored. However, some species grow quite large. The Goliath tigerfish (Hydrocynus goliath) and the golden dourado (Salminus brasiliensis) can both exceed 3 feet (1 meter) in length. At the other extreme, the smallest species, the Bolivian pygmy blue characin (Xenurobrycon polyancistrus), measures only about 0.5 inches (1.4 cm) long.