The Gemara quotes Rava as saying that there are two basic prohibitions of Hatmanah: "Hatmanah b'Davar ha'Mosif Hevel" -- "Hatmanah with an element that adds heat" which is even forbidden from Erev Shabbos, and "Hatmanah b'Davar she'Aino Mosif Hevel" -- "Hatmanah with an element which does not add heat" which is only forbidden when done on Shabbos. These two prohibitions are codified by the Poskim (see SHULCHAN ARUCH ORACH CHAYIM 257:1). An example of Mosif Hevel is totally wrapping a pot which is on a fire in towels, even before Shabbos. An example of the latter case is taking a pot off the fire and completely wrapping it in towels on Shabbos (Sefardim must be stringent to avoid even partial wrapping).

All the best,

Yaakov Montrose