On topic: I'd really like to know more about those other houses :huh:
The Houses we know:
House Hlaalu: Merchants, mostly. As such, they have a reputation of being dishonest, cowardly opportunists. Their capital city is Narsis, in the deep south. On Vvardenfell, it is Balmora.
House Redoran: Warriors, mostly. They hold honor, valor and bravery as the highest ideals, and are quite devout. Their capital city was Blacklight, on the north-west, but it was moved to Ald'ruhn by Bolvyn Venym.
House Telvanni: Mages only. Personal power, both magical and political, is all that matter in this highly individualistic house. Their capital city is Port Telvannis, on an island to the east of Northern Morrowind. On Vvardenfell, it is Sadrith Mora.
Those three houses have participated in the re-colonization of Vvardenfell, where they already had ancient holdings anyway.
The House we don't know much:
House Indoril: Priests, mostly. House Indoril is highly prestigious and closely tied to the Temple -- it is the House of Saint Nerevar and the Goddess Almalexia. So many of the Ordinators come from House Indoril that "Ordinator" and "Indoril" have become nearly synonymous; hence the "Indoril Armor" for Ordinator Armor. (Actual Indoril armor is different. As you know, all Great Houses have their own bonemold style that is worn by their guards, and a mere guard would never be allowed to wear an Ordinator's armor.) Its lands are in the east of Morrowind. Their capital city is Almalexia/Mournhold.
The House we know the least:
House Dres: Another house of traders like the Hlaalu, House Dres is mostly known for taking and selling slaves to all Morrowind. Its lands are in the south-east of Morrowind, bordering Black Marsh. Perceived as one of the most traditional-minded houses, many wild rumors circulate about them -- that they're ruled by vampires, that they still practice forbidden Daedric cults, and so on. It is probably mostly nincompoop, though.
The House that is no more:
House Dagoth: the Sixth House, whose ancestral holdings were on what is now Vvardenfell. House Dagoth was apparently a magic-oriented house, like the Telvanni, and they were the closest geographically to the Dwemer.
The House that was not a House:
House Dwemer: Since the Chimer had a political structure based on Great Houses, during the relatively short time in which they were allied with the Dwemer, the Dwemer nation was considered as being its own Great House, so some ancient texts will refer to House Dwemer. This was just for the Chimer's convenience.