The Horde is a union of several races of Azeroth, hostile to the Alliance. Warcraft movie shows history of the Horde from early beginning. Members of the Horde are: Orcs of Durotar, Tauren of Mulgore, Trolls of the Darkspear tribe and Undead Forsaken of Lordaeron. The membership of this union is not static and changed over time.
The Horde was originally founded by the Orcs and the Ogres who had fought against the Draenei race in Draenor. Trolls joined the union during the invasion of the Horde into Azeroth. During the first war forces of the Horde were affected by the dark power of the Burning Legion and had to fight against the Humans.
But after Alliance defeated Horde in the second war, dark power of the Horde was broken and Thrall could free his people from control of the Burning Legion. Free Orcs and Trolls started to search for a new place to live. They met with the race of Tauren in Kalimdor. Tauren offered them spiritual guidance and aid. Orcs saw that Tauren also respect strength and glory, so the races could form a union very fast.
Official Website: https://www.fightforthehorde.com/
The images below are taken from the posters pack of the Horde faction. This pack is available from the official website of the faction. But you can also see and download all the pictures from here.
Release Date: June 16, 2016