According to the material in the Campaign Guide of A Song of Ice & Fire RPG, there are four rankings of Houses in Westeros.
A Great House rules vassal knights over an entire region of Westeros. They owe their fealty directly to the King of Westeros. They command multiple banner houses and have significantly high House Abilities. Examples of a Great House would be House Stark, House Lannister, House Tully and House Arryn.
A Major House owes fealty to the Great House that rules their region. They command multiple banner houses, and have enough resources at their disposal to theoretically rival a Great House. Examples of a Major House would be House Frey, House Karstark, House Harlaw and [[House Mallister].
A Minor House owes fealty to either a Minor, Major or Great House within their same region of Westeros. The Minor House is the lowest ranking of lordship. They may command the fealty of multiple landed houses and ocassionally the fealty of another minor house. Examples of a Minor House would be House Clegane, House Westerling and House Flint of Flint’s Finger.
A Landed House owes fealty to a Minor, Major or Great House within the same region of Westeros. A Landed House is often referred to as a Knight House in parts of Westeros where knighting is common. Landed Houses have not been made Lord of a stronghold, but may have been charged with the duty of stewardship to their lord’s stronghold. Examples of a Landed House would be House Cassel, House Spicer and House Selmy.
A Landed House