A few more details.....
Eels spawn in the Sargasso sea in the middle of the Atlantic ocean. What's interesting is that the American eel and the European eel spawn in the same areas, but do not appear to hybridize. They go through several life stages, I can't recall exactly, but you end up with tiny baby eels. These are the ones that are of primary interest for the aquarist, as they adapt easier to aquarium conditions at that age. I put an eel, a male I believe into a tank with a pumpkinseed and a green sunfish. I had lots of rockwork. Consequently I didn't see him for almost 4 months, then one day he popped up eating pellets at the surface.
You can sex an American eel by its size and typical habitat. The females get much larger than the males, and tend to move into full freshwater. The males tend to stay in low to mild salinity levels, and are most of what the eels they sell for bait in my area are. The hardest part I've found with eels is getting them adjusted enough to feed. I've had more success in sand bottom tanks with rocks and stuff they can burrow under. They also spend some time in the sand, so make sure you have the correct substrate. Try to put them in the tank first, and let them get adjusted and feeding well before adding anymore fish. They are very shy at first, and maybe for a while, but I've only had one that just refused to come out at all. I would find it when cleaning the tank for months, and then it just disappeared. Which brings me to my next point. If an eel doesn't like it's environment, it will seek out a better one. They are magnificent escape artists, so a tight fitting lid with every single hole plugged is a must. This is where I lost all my eels eventually. I couldn't find out how they got out, but get out they did.