Eels are characterized by an elongated and cylindrical body, reminiscent of a snake. They do not have pelvic fins, and in several species the pectoral fins are also absent. In those that do have pectoral fins, these are placed in an approximately half-lateral position. Both the dorsal and anal fins are confluent with the caudal fin, thus forming a single continuous fin, which runs along most of the animal's length. Their skin is generally devoid of scales, that is to say they have bare skin, although there are certain species that do have scales and these are usually embedded in the skin. The gill openings are generally narrow and located in a slightly backward position.

Predatory and generally active at night, Eels have highly developed teeth, with teeth all over the mouth. Many species specialize in entering small openings, cracks and cavities, although there are other preferences in terms of habitat such as species that live in the blue (pelagic species) or digging into soft substrates. They swim by making undulating movements that travel along their body, and as a curiosity, they are able to swim backwards by simply reversing the direction of the undulations.

Among the Eels we find species that are known to all, such as Freshwater Eels, Morays Eels and Conger Eels. There are approximately 940 species within the order of Eels, which are distributed in 19 families and 159 different genera, most of which are marine. Eels range in size from 5 cm for Monognathus ahistromi to 4 meters for Strophidon sathete and weigh from 30g to over 100 kg (Conger conger).

Eels begin their life cycle as flattened, translucent larvae that later undergo a process of metamorphosis into adult individuals.