An Iron Skillet is, as one might imagine, a skillet made of cast iron. They typically have either tall sides, or no sides at all. They are heavy, transfer heat fairly well, and can be used for almost any kind of cooking.
The most important property of an iron skillet is that it is made of iron. What this means from a practical standpoint is that it tends to soak up whatever you put in it. For this reason, you must never wash any iron cookware with soap. Also, you must always use some kind of shortening to ensure that foods do not stick to the pan; Butter, oil, or grease. Over time the pan will take on these materials, at which point we say the pan is seasoned.
To clean an iron skillet, you can drain any oil, and then put the still-hot pan under a stream of water and scrape it with a metal spatula. The water will break food loose, and the scraper will remove it. If baked-on food is unresponsive, you can fill a high-sided skillet with water and boil it, which will break it up, but may unseason the pan. After doing this you should add a little oil. If you leave the pan wet (or too wet to dry fairly rapidly) it will rust, so do not neglect the seasoning.
Iron transmits heat fairly readily, and so will cook foods reasonably evenly. It seems to cook more evenly than revereware (cookware with a copper bottom for heat transfer) but not as well as cookware which uses air passages in its base.
The "Iron Skillet" is also a restaurant chain generally found in truck stops. They have a breakfast buffet, and generally decent fare at a basically low price.