The Eight Noble Houses of Halruaal represent the most powerful Wizards that have ever ruled the Magisterium. Each of the Houses specialize in a particular form of Magic (i.e. Abjuration, enchantment, necromancy). Each House is home to usually an Archon (the respective head of the family and most powerful wizard of their specialization) and their Lord/Lady Paramount.
These Archons rule over a region of Halruaal. Each House holds a number of tight knit houses which in turn governs a number of lesser noble houses that once served as Captains and Lieutenants when the Houses initially arrived in Halruaal. House Udaku has been the reigning Royal Family since they first arrived in Halruaal. the most powerful of House Udaku has been crowned as the First Archon, with their spouse/partner typically given the title Lord/Lady Supreme.
To denote the difference between the Ruling Eight Houses/Clan and "minor" nobility common etiquette holds that when talking about the big eight one uses "Houses" or "Clan" (note the capital letter when discussing their vassal, or minor nobility, one uses "house" (note the non-capital letter). Major vassal houses are either incredibly old (possibly having arrived with the original Archon of House Aetius) or are home to incredibly powerful wizards. Minor vassal houses are usually - though not always - relatively new houses. These are established in the last couple of centuries (usually no older than 1500) or they are houses that don't have particularly powerful wizards. Declining houses are houses that have either fallen out of favour, or have not produced powerful wizards in the last couple of generations.