The Klingon Houses are families present within Klingon society.
The Klingon Empire's territory is divided into numerous fiefdoms dominated by Houses - both large and small. These families rule their lands directly with absolute territory and can consist of anything between small planetary regions to entire planets with the more powerful Houses controlling several planets. This shows itself as a petty group of Houses that are bound together for mutual benefit that resemble some ancient societies on Earth rather than an intergalactic power.
Within each House, there exists their own military force, training facilities, arsenals and even shipyards maintained by its occupants. A House that wins many battles and controls great swathes of land, the more successful it is which means even more warriors will join its ranks. As such, the House is the most important aspect in a Klingon warriors life and it defines their allegiance. This means that its often that a Klingons loyalty is stronger to his House than his dedication to the Empire itself. This means that every honorable warrior serves his House with undying devotion. (Decipher RPG module: Aliens)