Ork Clans are cultural groupings of Orks rather than actual communities, each embodying a distinct Orkish philosophy. Clans exist outside the organization of tribes - where tribes are inconstant, breaking apart and reforming in war, Clans are constant and enduring.
There are countless tribes and warbands throughout the galaxy, but only six significant Clans. Each has its own distinct character, culture, colours, markings, and ways of waging war. The Clan distinctions are possibly genetic, engineered into the Orks by the Brainboys of the distant past.
A large tribe usually consists of Orks of different Clans. Orks tend to form tribes and warbands with others who follow the same clan, but different groups tend to be lumped together after suffering casualties during a Waaagh.