There will be spoilers for Warlords of Draenor in this post
So you may be confused right now. With the soon to come introduction of the Iron Horde, we've entered a situation where we've got several different Hordes. There's the Old Horde, the original Horde that invaded Azeroth from Draenor. There's the New Horde, or Horde 2 if you prefer, the Horde put together by Thrall after he liberated the orc survivors from the Second War, leading them out of the internment camps and to the shores of Kalimdor. Along the way they picked up the tauren and Darkspear trolls as allies, and eventually the Forsaken, blood elves and Bilgewater goblins joined their ranks. This Horde is the Horde most players are still a part of. Meanwhile, at the same time, there was the Dark Horde, consisting of Blackrock orcs under the command of Rend Blackhand and his ultimate master, Nefarian.
During his reign as Warchief of the Horde (the New Horde, to be specific) Garrosh Hellscream recruited both the Blackrock and Dragonmaw orcs to join the Horde. But at the same time, he grew disenchanted with the other races of the Horde - goblins were too greedy, blood elves too mercurial, undead too loyal to Sylvanas and distasteful to his sensibilities, the Darkspear too loyal to their Vol'jin, and the tauren ultimately too bound to their own code of honor. It didn't help that he'd killed Cairne Bloodhoof and further alienated the tauren clan leader Baine, Cairne's son, with his actions. Soon, Garrosh found himself leading a Horde he didn't much trust or care for.