The canals of the city of Vivec
Some time after the Chimer* followed the prophet Veloth to Resdayn,** the formerly nomadic tribes begun to settle down permanently. From these settled groups of tribes arose the more formalised House structure that now dominates Dunmer society. Great Houses function as immense families that operate politically, economically and philosophically as imposing and (generally) unified forces. Morrowind is de facto ruled by a Grand Council of the most powerful Great Houses.
A murky swamp, with a moody Silt-Strider
House Dres: The treacherous and traditionalist slavers that control the food bowl of the Dunmer culture.
Indoril Ordinators on duty
House Indoril : The staunchly traditional defenders of the faith, from whence came Nerevar Moon-and-Star.
A young Telvanni mage collecting ingredients
House Telvanni: the mysteriously aloof mage-lords. Telvanni are extremely isolationist, not caring who claims what, provided they have enough sense to stay out of the way.
A young adventurer setting out from the city of Vivec
Sadly we don't know anything about House Sadras at all. Their ascendancy does, however, confirm that there are more houses in Morrowind than the great five. House culture is filled with lots of smaller Houses, lacking in the power of the Great Houses, but no less rich in tradition and history.
* The race of mer who would latterly become the Dunmer
** The land more commonly known as the province of Morrowind
*** The year the Nerevarine came to Vvardenfell. Following which, House Dagoth was destroyed.
Did you know that for a brief period the Dwemer of Resdayn were referred to by the Chimer as House Dwemer? Have a Great House you'd particularly like to start examining first? Let me know in the comments!