The Horde Expedition oversees all the forces of the Horde in Northrend including:
Their Alliance counterpart is the Alliance Vanguard. If no other Championing tabard is worn in Championing dungeons, Horde Players will gain reputation with this faction instead.
Half of the reputation gains from the Horde Expedition's four sub-factions will also count towards your reputation with the Horde Expedition. Some quests (including the Grizzly Hills daily quest [74P Daily] Blackriver Brawl) give Horde Expedition reputation directly.