The Trading Companies are the de facto Factions encountered in Sea of Thieves.
Any pirate worth their treasure hoard knows that to survive in the Sea of Thieves, they need the backing of a powerful group. Those groups are the Trading Companies, a group dedicated each to their particular mode of "trade" on the high seas.
Every outpost from Ancient Spire to Galleon's Grave has space for a representative from the Companies, allowing pirates of all backgrounds to interact and pledge their allegiance
As pirates performed appropriate tasks, i.e. Voyages, for a Company, they gain Reputation with that Company, paving the way for shinier rewards and more risky & more rewarding Voyages.
As of the Final Beta, only two Companies are currently in operation: the Merchant Alliance and Gold Hoarders, though the Order of Souls is expected to open its doors on launch day.