The Horde is one of the two major political factions in Azeroth, its counterpart being the Alliance. The Horde is a faction founded by Orcish refugees from Draenor and have survived in the hostile world of Azeroth by forging alliances with local races such as the Tauren, the Trolls and the Forsaken undead.
After the position of Warchief was taken by Garrosh Hellscream following Thrall's disappearance not long after the Northrend Campaign, the Horde began to slowly disintegrate due to Garrosh's warmongering. Garrosh kicked the Blood Elves out of the Horde, Sylvanas Windrunner began to ignore Garrosh's authority and the Magatha Grimtotem deposed Baine Bloodhoof as High Chieftain of the Tauren and took over Thunder Bluff.
Eventually, civil war erupted within the Horde after the Troll Empire seceded from the Horde and became the forefront of the rebellion known as the True Horde. Led by Thrall and backed by the Blood Elves, the Forsaken and the Tauren Bloodhoof tribe, the rebels waged war against Garrosh's followers, who called themselves the Iron Horde. The rebels ultimately overthrew Garrosh Hellscream, who was slain in a duel with Thrall, who made Vol'jin the new Warchief of the Horde.