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.
This led to his deliberately favoring Blackrock and Dragonmaw orcs (and any other orcs which would swear loyalty to him, personally, over the institutions of the Horde proper) and thus was born what Hellscream called the True Horde. This Horde shared many t hings with the Horde Thrall had led - it was headquartered in Orgrimmar, it laid claim to the loyalties of all the Horde who'd served during Thrall's reign. In the end, these two Hordes clashed and Garrosh's Horde proved too limited in scope, lacking in diversity, especially with the forces of the Alliance also engaged in their destruction. But this isn't the end of Hordes we'll have to deal with.
When Garrosh Hellscream escaped from his confinement with the help of a group of loyal followers (including Warlord Zaela, head of the Dragonmaw clan) he appeared to travel back into the past of his shattered homeworld, to before it was destroyed by Ner'zhul and his portals. But while it's become apparent to us all that it's not the exact same world as the Draenor we knew - Ner'zhul's wife Rulkhan still lives, Akama is not an anchorite but a proud draenei Vindicator, Gul'dan seems to have directly contacted the Burning Legion and drank the blood himself first, instead of Ner'zhul being duped into first contact with Kil'jaeden - it's still clear that Garrosh took steps to make sure that Grommash Hellscream would not drink the blood of Mannoroth, and instead, in the slaying of the pit lord and the use of Gul'dan to fuel a time and space bending superweapon, the two Hellscreams created an entirely new kind of Horde - one that placed its faith neither in the demonic magics of the Old Horde nor the shamanism of the New Horde's orc contingent, but rather the ironmongery and black smoke and soot belching furnaces of an industrial age brought to this Draenor by Hellscream and his allies.
The problem with the Iron Horde is this - we see Hellscream escape to the past with Kairoz, but many other allies of Hellscream managed to escape Hellscream's liberation. These included goblins and many orcs that were at least familiar with the devices that made up Garrosh's True Horde war machine. Furthermore, these allies were riding infinite dragons. Zaela's presence and that of the Blackfuse Company implies that the infinite dragons have brought Garrosh's allies to the alternate Draenor and allowed the Iron Horde access to this technology.
The Blackfuses created the Grimrail Depot and other structures (it's likely that they had a hand in the massive Blackrock Foundry as well) and it serves to show the line of descent. The Iron Horde is the spiritual successor to Garrosh's True Horde. They use similar technology, they share a similar racial identity (primarily orcs, with enslaved gronn and ogres and mercenary goblins providing technology).
So we have the following Hordes, and the following means to differentiate between them.
- There's the Old Horde (called just The Horde during its period of existence), a group of orcs who drank the Blood of Mannoroth and were united under first Warchief Blackhand, and later Warchief Orgrim Doomhammer, as a force of conquest seeking to destroy and dominate Azeroth. Secretly controlled by the Shadow Council under Gul'dan, this army of orcs nearly conquered the Eastern Kingdoms before Gul'dan, seeking ultimate power, betrayed them and was destroyed by demons at the Tomb of Sargeras
- The Horde of Draenor (also just called The Horde) was a group consisting of the survivors of the Old Horde as well as those orcs left behind on Draenor because Gul'dan believed them too wild or treacherous for Blackhand to control them. Ultimately led by Ner'zhul, Gul'dan's former teacher and the original force behind the creation of the Horde and the war with the draenei (due to his being misled by Kil'jaeden), this Horde was ultimately destroyed when Ner'zhul used magical artifacts to open multiple portals in an attempt to escape the slowly dying Draenor, destroying it in the process. Many orcs managed to escape into Azeroth rather than share this fate, Grom Hellscream among them.
- The New Horde (also called just The Horde during its existence) was created some twenty or so years later by Thrall, son of Draka and Durotan, an orc raised by Aedelas Blackmore to be a slave gladiator and ultimately the head of an army of orcs to help Blackmore conquer the human kingdoms of Azeroth. Instead, Thrall escaped, reconnected with the orcish heroes Orgrim Doomhammer and Grom Hellscream, and started freeing the orcs in internment camps across the Eastern Kingdoms. Upon Doomhammer's death, the elder orc proclaimed Thrall the Warchief of his people. Thus, the New Horde is a direct descendant of the Old Horde, as Doomhammer was Warchief of that Horde. While the Old Horde was ultimately ruled by the Shadow Council and Gul'dan and was corrupted by warlock magic, the New Horde was a far more shamanistic endeavor even before Thrall met the Darkspear trolls and Cairne Bloodhoof's tauren. These peoples joined his Horde, helping complete its transformation away from the legacy of the Old Horde. Ultimately the New Horde utterly rejected the Burning Legion, going so far as to fight against them atop Mount Hyjal.
- The Dark Horde served as a rival to the New Horde. Founded by Rend and Maim Blackhand, sons of the first Warchief of the Old Horde, this Horde consisted of members of the Black Tooth Grin, Blackhand, and Dragonmaw clan (most of the Dragonmaw on Azeroth remained in Grim Batol at the time) and served the Black Dragonflight in its war with the Dark Iron Dwarves and Ragnaros for control of Blackrock Mountain. The Spirestone Ogres, Smolderthorn trolls and others served Rend and Maim as members of this Dark Horde, and when Nefarian came to Blackrock Mountain he took control of the Dark Horde. Maim Blackhand died in battle with the Dark Irons. It's important to note that when Ner'zhul's Horde of Draenor came through and sought aid from Rend and Maim, they refused him - ironically, Ner'zhul would ally with Deathwing, Nefarian's father, and later Rend would come under Nefarian's control. Following the death of Rend and the defeat of Nefarian, Eitrigg and his son Ariok would defeat their former clan and so, the Blackrock orcs would join Garrosh Hellscream's New Horde, and later, his True Horde.
- The True Horde, formed by Garrosh Hellscream, was equal parts Dark Horde (members of the Black Tooth Grin and Blackrock clans), New Horde (any orcs that would swear allegiance to Garrosh over all) and formerly independent orcs like the Dragonmaw of the Twilight Highlands (distinct from those Dragonmaw that remained loyal to Rend Blackhand, or the ones who remained behind on Outland and served Illidan Stormrage) with some goblin allies. It's difficult to know exactly when the New Horde stops and the True Horde begins - prominent members of the True Horde like Garrosh and Nazgrim were at one point part of the Old Horde, and in Nazgrim's case may never have actually realized he'd joined a new organization. The main differences between the True Horde and the Old Horde are in the xenophobia of Garrosh's True Horde, its willingness to use darfk shamanism to coerce or punish the spirits and elementals into obedience, and its emphasis on personal loyalty to a cult of personality over that to the Horde itself. The True Horde may be said only to have definitely existed once the Darkspear Rebellion forced uncommitted members of the Horde to take a side, and it ceased to exist with the defeat of Garrosh.
- Finally, the Iron Horde, created by Garrosh and his allies, but led not by Garrosh but by Grommash Hellscream, this Draenor's version of the orc who drank the demon blood first and ushered the orcs of the Old Horde into slavery. This version of Grommash would, thanks to Garrosh's influence, refuse the blood and unleash the militarized power of an industrial war machine on the Pit Lord Mannoroth, destroying him and ushering in an age of conquest for the orcs of his Draenor.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.