The crest of the New Horde.
High Overlord Varok Saurfang
The New Horde is a collective entity founded by Thrall that originally consisted of the orcs of Orgrimmar, the trolls of the Darkspear Tribe, the tauren of Thunder Bluff, and the undead of the Forsaken. It has since grown into a major power in the world of Azeroth, and have since been joined by the Blood Elves of Quel'Thalas, the Goblins of the Bilgewater Cartel and the Huojin Pandaren.
The Horde in Kalimdor, which acts as the bulk of the Horde's forces, serve under the Warchief's leadership. They include most of the Orcs, the Tauren, the Darkspear Tribe, the Goblins of the Bilgewater Cartel, the Huojin Pandaren, and a number of less prominent races within the Horde - such as the Stonemaul ogres.
The Horde in the Eastern Kingdoms, comprising of the Forsaken and the Sin'dorei, act with more independence from the Kalimdor Horde but ultimately answers to the Warchief out of necessity for the Horde's support.
These races all see the Horde differently. Some see it as an instrument of vengeance, hellbent to carve out a future for its people. Others see it as a refuge for those who need shelter, a commonwealth of the downtrodden, built for mutual support. While some view it as an alliance of convenience, a confederacy of unlikely partners free to pursue their own agendas. Whatever their motivations, they are united to claim Kalimdor and the Eastern Kingdoms for themselves in order secure key provisions and supplies needed to support their people, and forcibly remove Alliance incursions from their own lands. This Horde is the result of the actions in Warcraft III: Reign of Chaos.
The orcs of Durotar follow High Overlord Varok Saurfang. Their culture is split mostly between the orcs' warrior spirit and their shamanistic roots, which has recently wrought on the conflict between the younger and older generations. Dedicated to carving out an existence in a world that has come to largely revile them, the orcs have recently turned their attention to reinvigorated conquests. Ashenvale and Stonetalon are among their first targets, and ultimately Warchief Hellscream intends to unite the entirety of Azeroth under a single banner.
The Darkspear of the Broken Isles have a very savage and dark history studying voodoo arts and worshiping the loa, however after Thrall had saved their tribe from extinction, Vol'jin son of Sen'jin pledged his tribe to the Horde. Some trolls have now begun to study shamanism and more recently druidism, some of these trolls even mix shamanism with voodoo arts, most of the members of the tribe have or are attempting to give up the ways of cannibalism and the other traits of their race's darker history. After retaking much of their territory from the treacherous Zalazane, the Darkspear have been invited to join a revitalized troll empire under the Zandalari's leadership - though Vol'jin has declined this offer.
The noble tauren of Kalimdor is a race that had hunted the lands of central Kalimdor for generations, however, when Thrall and his orcs had arrived onto the lands of Kalimdor they had already been driven near extinction by the marauding centaur that rampaged through their lands. Under the leadership of their high chieftain Cairne Bloodhoof and the young warchief Thrall, the tauren managed to drive back most of the centaur. Since then, Cairne has pledged his people to the Horde. After Cairne's death, the young Baine Bloodhoof has arisen to take his father's place and lead the tauren into a brighter future. With the recent Alliance incursion into the Barrens, the tauren have created a mighty gate to protect their homeland of Mulgore. Even if their Horde allies turn down dark paths during this war, the tauren have resolved to be the light that guides them through it.
The Horde of the Eastern Kingdoms was initially next to non-existent, with only the Frostwolf orcs and other smaller forces in operation around the continent. In recent years, however, the Horde's presence has been bolstered by the inclusion of the Forsaken of Eastern Kingdoms and the Elves of Quel'Thalas. While considered members of the Horde, their bonds with the western Horde are perhaps not on the same level as the orcs, trolls, and tauren.
The Forsaken were once part of the Scourge, forced into mindless servitude under the Lich King. After the Lich King's grip began to weaken, the banshee queen Sylvanas Windrunner broke free from his will, and went on to both free and rally other undead beings to join her own empire of undeath, dubbed the "Forsaken." Under Lady Sylvanas' leadership, the Forsaken used any means necessary to conquer the fallen kingdom and remove both the dreadlords and other foes threatening their dominion. After expelling the Scourge from capital city and massacring the last remaining holdouts of the Alliance, they took over the catacombs of the former capital, renaming the city "Undercity."
With the backing of the tauren, Thrall warily accepted the Forsaken into the Horde, though this alliance of convenience has been tested - great betrayal at the hands of Grand Apothecary Putress has put the Forsaken on a tight leash, made tighter by the new Warchief's less lenient stance on their modus operandi.
The Sin'dorei of Quel'Thalas preside over the other major kingdom of the Eastern Horde. After the Scourge onslaught essentially culled the high elven race, with over 90% of their people killed, the remaining 10% vowed to avenge the destruction of their once-great nation and dubbed themselves the "blood elves" in honor of their fallen brethren. After retaking their homeland with the volatile new powers the magisters advocated, the kingdom of Quel'Thalas was offered aid from an unlikely source - the Forsaken, whose queen, Lady Sylvanas, had lost her life in her attempt to protect Quel'Thalas from the Scourge. With their assistance, the blood elves were able to fend off the lingering Scourge in the kingdom and were able to enter the Horde as an equal nation of its own.
Quel'Thalas suffered another dark hour in recent years when its twisted scion Kael'thas Sunstrider attacked his homeland in a mad bid to usher the Burning Legion's master into Azeroth. He was put to death for this on Quel'Danas, and Lor'themar Theron has thus become the sole leader of the blood elven people.
The cunning Bilgewater Cartel goblins from Kezan, one of the most (if not the most) powerful goblin cartels that dominate trade in the south seas, have recently joined the Horde. After Deathwing assaulted Kezan and the goblins were forced to flee, their first escape ships were attacked by an Alliance fleet. With a common enemy, the goblins assisted the captive former Warchief Thrall in breaking his bonds and obliterating the Alliance on the Lost Isles. Their future was tested when their leader, the angered Trade Prince Gallywix, attempted to defeat Thrall and take over Azshara for himself, though this ultimately ended in failure. Thrall mercifully allowed him to retain his position as Trade Prince, and thus the Bilgewater goblins were accepted into the Horde.
These pandaren follow the path of the Huojin, a philosophy among the Pandaren that inaction is the greatest injustice, and that there is no shame in defending one's homes and loved ones regardless of cost. That pandaren who follow the path of Huojin are allies of the Horde and are represented and led by the intrepid Ji Firepaw, a staunch adherent of this philosophy
The Huojin are quick to act and quicker to fight for what they believe in. Impulsive and practical, they feel that the ends can justify the means.
The Nightborne is a powerful and mystical race of night elves who live in Suramar. Since the city was first separated from the rest of the world over 10,000 years ago, they have evolved by the Nightwell into an arcane-enhanced version of their former selves.
After the death of Grand Magistrix Elisande and the end of the Third invasion of the Burning Legion, the Shal'dorei wished to rejoin the world. After a harsh exchange between Thalyssra and Tyrande Whisperwind, and an offer of mutual friendship from Lady Liadrin, the Shal'dorei leader decided that it would be best to ally with the Horde, who share ideas similar to those of the nightborne.
The Highmountain Tauren are found in the region of Highmountain on the Broken Isles. Divided into four tribes (Highmountain, Rivermane, Skyhorn, and Bloodtotem), they are the direct descendants of the tauren led by Huln Highmountain who assisted the Kaldorei Resistance during the War of the Ancients. During the war, Huln received a blessing from Cenarius, so that his people would forever have the demigod's favor, granting them the Horns of Eche'ro: moose-like antlers.
During the third invasion of the Burning Legion, the drogbar, who had previously lived in peace with the Highmountain, rose up and stole the sacred hammer, shattering the unity of the four tribes.
After the defeat of the Burning Legion, the Chieftain Mayla Highmountain and her people visited Kalimdor when Chieftain Baine Bloodhoof invited them to Mulgore for a feast.
The Mag'har orcs are a noble race with a shamanistic tradition cultivating a mighty tribal society that was centered around survival, regulating themselves through ritualized combat and personal honor. They refused the dark bargain of Gul'dan and banded together to drive out the Burning Legion, after their victory, the clans stood united as one tribe under the leadership of Grommash Hellscream.
After their escape from Draenor and their Warchief's sacrifice against the fanatical crusade of the Lightbound, the clans pledged their loyalty to the Horde.
The Zandalari trolls are a powerful and superstitious tribe descended from the first troll tribes, they settled on the highest peaks of this mountain range populated with loa, where they built crude shrines that eventually became the massive temple city of Zuldazar. These trolls are scholarly and value knowledge; they preserve troll history and work to further the goals of troll society as a whole. They are also masters of war and strategy, having led the trolls in many battles before, and having for many millennia tamed massive dinosaurs. Currently, they possess one of the most powerful navies on Azeroth.
The Orgrimmar Army is the collective offensive and defensive military force of the orcish nation of Durotar. Established by Warchief Thrall after the founding of Durotar in the aftermath of the Third War, it boasts thousands of troops of varying backgrounds from throughout the New Horde. Orcs, Tauren, Goblins and Darkspear Trolls make up the majority of the army while Forsaken, Blood Elves and Pandaren make up a small minority. The army has fought in every major conflict that involved the New Horde since its foundation.
The Thalassian Army is the standing military of the Kingdom of Quel'Thalas. Primarily made up of Farstriders and Blood Knights, the Kingdom's military is commanded by the Ranger-General of Silvermoon.
The Forsaken Army is the second most numerous united fighting force in Azeroth. It numbers almost a million soldiers. Most of the Army consists of heavy infantry, who are highly disciplined and rely mostly on their share toughness to defeat most opponents. The majority of warriors are former Lordaeronian soldiers, lead by long-dead officers.
The Duskwatch is the primary body responsible for maintaining law and order in the city of Suramar. Boasting a wide array of skilled mages and warriors alike, the Duskwatch represents the bulk of Nightborne armed forces.
The New Horde Navy is the naval force of the New Horde. Though it is the Horde's answer to the Grand Alliance Navy, it is vastly smaller than it's counterpart, being comprised mainly of Orcish man-o-wars, Thalassian Destroyers, and Forsaken battleships. However, it boasts strong vessels and prides itself in its strength of numbers and soldiers.
The Forsaken Navy is the naval military force of the Forsaken of Lordaeron. It is one of the largest and most capable navies of Azeroth, though does not succeed the likes of the elves or that of human kingdoms.
The Thalassian Navy is the naval military force of Quel'Thalas, led by Ranger-General Halduron Brightwing. While small in comparison to other Horde naval forces, they have been known to be a strong force to help their allies.
The Golden Fleet is the naval military force of the Zandalari Empire, led by the God King Rastakhan. Regarded as one of the strongest naval forces in Azeroth, the Zandalari joined with the Horde following the faction's help in the Zandalari homeland, Zandalar.
Partially adapted from Wowpedia