During the American Civil War both sides were trying to make use of the comparatively new concept of ironclad vessels. The Union army often compromised and built a lightly armored steamship jokingly called a tinclad; the armor was iron, but there simply wasn't much of it, in either thickness or total coverage, and these boats simply weren't as impressive as the European ironclads.
But there also wasn't much need for an impressive warship in the Civil War. Most of US naval warfare took place in coastal waterways and rivers, and a large, high-riding sailing ship wasn't wanted. Large sailing ships work well on the high seas, where there's generally a good wind and lots of room for maneuvering. A high-riding ship provides a very good target, but you need a ship that can weather the high waves of a stormy sea. On the river you didn't get such high waves, you couldn't count on room to tack against the wind, or steady wind, or that if you were becalmed you were probably becalmed safely away from any shore-bound gun emplacements or saboteurs with rowboats. Also, lighter ships could go further into the shallows, which the coast and rivers have a lot of, without foundering. And also, supply lines were pretty good and the ports were nearby, so coal powered ships, already used in war in Europe, were easier to provide for in the states.
So, what do you do if you're fighting a river war with your neighbors? You take any powerful steam ship (a paddle steamer, at this point in history), take off anything that sticks too high up (easy to shoot at), build a casemate for your guns, and add a bit of iron plating anywhere that looks important and easy to armor, usually especially the casemate. These ships were not armored to withstand a cannon battle so much as to protect the crew from gunshot and shrapnel; at this time the Southern states were constructing cottonclad warships armed with sharpshooters, so the bar was set a bit lower for the Americans than it was for the European navies. However, tinclads were most often used in support roles, such as escorts and patrol duties.
It would be remiss not to mention that tinclads were much cheaper and easier to engineer than were ironclads, and much quicker to source. This may also contributed to the wide acceptance of such a derogatory name as tinclad.