Could be anything. If you're unlucky, it's Anguilla or Synbranchus spp. Neither of these genera contain species that are suited to the average home aquarium. If you're lucky, it's just a spiny eel. Again, not an easy fish to keep, but far from impossible if you have a tank with a secure lid, sandy substrate (for small species especially), and no fishes likely to compete with it for food (such as loaches and catfish).
Anguilla spp. will look like standard European (or American) eels. Snakelike, pectoral fins very small. Dorsal and anal fins each form long fins along most of the length of the fish and merge with the tail fin. Big, predatory fish that will eventually escape.
Synbranchus spp. have virtually no fins at all, and the gill slit is runs from one side to the other under the throat, as if the fish has had its throat slit. Odd, incredibly nasty fish that will eventually escape.
Spiny eels have, as their name suggests, a row of spines along the back. The head is usually quite long and pointed, in many cases with a mobile "trunk" at the end. Often have quite nice colours. Fun fish, but usually escape and end up dried on the floor.
Cheers, Neale