The electic fishes are those who can generate or detect electric fields. A fish that can generate an electric field is said to be electrogenic, a fish that can detect an electric field is said to be electroreceptive. Usually electrogenic fishes are electroreceptive too but the opposite is not always true.
Electric eel, Electrophorus electricus Image source
The electrogenic fishes can use their power thanks to a biological structure made of muscle tissue and nerves that can generate an electric potential difference when contracted. This structure is said electric organ and it's usually located on the tail.The electrical output of the organ is called the electric organ discharge.
Electric catfish (Malapterurus electricus) Image Source
Among that kind of fishes are many sharks, rays and catfish. All of them got the ability to detect elecrtic fields and they use this ability to hunt or to orient themselves in the space.
Marbled electric ray, Torpedo marmorata Image Source
The electrogenic fishes are 400 species located in many orders. Most of them live in freshwaters, the only exception are some ray fish that live in saltwaters.
Lesser electric ray, Narcine bancroftii Image Source
There are two kinds of electrogenic fish: strongly electric fish and weakly electric fish. The strongly electric fish are the one that can generate and electric organ discarge (EOD) that range from 10 to 600 volts with a current up to 1 ampere. That EOD is useful to stun prey or to escape from enemies. Known strongly electric fish are electric eels (Electrophorus electricus), electric catfish (Malapteruridae family) and ray fish (Torpediniformes).
Peters' elephant-nose fish, Gnathonemus petersii Image Source
The weakly elecrtic fish can generate and EOD lower than 1 volt, useless to stun any prey or enemy but useful to find prey in the mud or to comunicate with other fish of the same species. Known weakly electric fish are the Peters' elephant-nose fish (Gnathonemus petersii) and the black ghost knifefish (Apteronotus albifrons) both commonly bred in freshwater aquariums.
Bblack Ghost Knifefish, Apteronotus albifrons Image Source