Anguilliformes - true eels
There are many fish which go by the name of 'eel', but the true eel, the freshwater variety are all members of the family Anguillidae. They barely need description, being without exception long, slender fish, usually without scales, and with long anal and dorsal fins, giving the appearance of being continuous along the fishes' body.
There are 16 species of freshwater eels across the world, all are predatory, feeding on fry, freshwater crustacea and smaller fish. Their young are hatched in seawater - many species, especially North American and European eels, make a migration to the Sargasso Sea to spawn, and so determined are they to make this journey that they may even cross dry land in order to reach the sea. The fry grow in the sea, and slowly move toward land, metamorphosising into the form known as elvers, which then swim back to fresh water.
Because they are scaleless, they are capable of absorbing a certain amount of oxygen through their skins, provided the skin is damp. It is this adaptation which enables them to make overland journeys, not just to spawn, but to move to new waters should their own river dry up.
Their frog-like skin and sinuous nature has given rise to the expression 'slippery as an eel', an apt description of many people who will twist and turn to avoid answering questions, taking responsibility or making a stand. Certainly, having watched an angler landing an eel, I can vouch for the difficulty in handling them.
So what do eels eat? They are carnivorous, so will feed on pretty much anything smaller than themselves - insects, crustacea and snails, in addition to smaller fish.
Eating eels? Yes, they are a good source of food, not just to other wildlife (otters being rather fond of them), but also to humans. In London, the traditional dish of jellied eels is still enjoyed by many (though not this boy), and they are also enjoyed throughout the Far East (think sushi) and in South America.
Other varieties of eel are:
Anguillidae - freshwater eels
Muraenesocidae - pike conger eels
Nemichthyidae - snipe eels
Congridae - conger eels
Sadly, the electric eel is not a true eel, but belongs to the order Cypriniformes.