Introduction to the Horde

[Adara]

((most of this information is taken for wowwiki website))
"The Horde is one of the two political factions in Azeroth. Orgrimmar is the obvious main city of the Horde. Thrall is the undisputed ruler, and he stands as Warchief over the entire Horde. He has dominion over the taurens and trolls alike. Despite their somewhat monstrous appearance, the majority of the Horde are not evil. On the contrary it is composed of diifferent factions and individuals who haves a wide range of views and values.
Orcs and taurens believe in redemption more than most other races on Azeroth and are willing to give almost anyone a chance, regardless of reputation. The Revantusk seem to have even accepted, (if not at least tolerate) their ancient enemy the blood elves. Largely because of these beliefs, a number of mortal races and many diverse factions can be found in service in the Horde."

The Two Factions of the Horde:
"One group lives mostly in Kalimdor, and follow mostly Thrall's leadership. It includes most of the orcs, the tauren, the Darkspear trolls and some ogres. They are united by a shamanistic culture, honor, and their respect for Thrall, who they consider their hero and affirmed leader.
The other group lives mostly in the Eastern Kingdoms. It includes the Forsaken, the blood elves, the Grimtotem tauren, and sometimes the evil orcs of the Dark Horde
The Forsaken are allied with the Horde but serve only themselves. They are much darker than their allies from Kalimdor, do not necessairly consider Thrall as their leader and certain members of their race do not care about honor, morality or even other races. Not all Forsaken are evil, but many are, and other races definitely view them as such. A non-evil Forsaken must work hard to prove his neutral (or perhaps, good) intentions. Few good Forsaken exist, but many evil ones do, and their leadership is definitely up to nefarious ends. They are current ruled by The Banshee Queen, Lady Sylvanas Windrunner.
Blood elves despise most other races, but they are coming to accept that the Forsaken are different, or at least share the same goals. Most blood elves are not insane or evil - they just choose to fight fire with fire. They were suffering for lack of the Sunwell; they all needed -- and still need -- to feed on arcane energies, even though most of them aren't spellcasters. The most powerful blood elf spellcasters are insane, as the magic they wield is corrupting The western Horde recognizes the blood elves as dangerous, volatile and ultimately destructive.
Even if many Forsaken and some blood elves are evil, Thrall's horde has kept them mostly in check."

Read up on each of the races of the Horde:
Orcs
Trolls
Tauren
Undead (Forsaken)
Bloodelves