An eel is a long, slender fish that belongs to the order Anguilliformes. They are typically found in freshwater or saltwater habitats and have elongated bodies with long dorsal and anal fins. Eels are known for their unique ability to swim in both water and on land due to their muscular tails, which they use to propel themselves through the water. Some species of eels are also capable of changing their sex during their lifetime, a phenomenon known as sequential hermaphroditism.