An eel is considered to be a fish. They use gills to breathe and spend most of their lives under water. A true eel is a member of the fish order Anguilliformes.
The Anguilliformes order of fish is broken down into four suborders, 19 families, 110 genera, and approximately 800 species. Some other fish are mistakenly called "eels," such as the electric eel, but they are not members of the Anguilliformes order. True eels spawn in the ocean and make their way back to fresh water, whereas the electric eel is a freshwater fish that lays its eggs in freshwater.