I will give a brief account of the player races of Azeroth in three logical sections, grouped according to three unique circumstances. These are:
- The Seed Races, as created by the Titans;
- Aliens, which more or less were introduced into the Azerothian environment;
- The long-eared and bovine races, whose ultimate origins are not recorded by any surviving written sources.
The Seed Races
When the Lord God made the universe, there were no plants on the earth and no seeds had sprouted, because he had not sent any rain, and there was no one to cultivate the land...
Then the Lord God took some soil from the ground and formed a man out of it; he breathed life-giving breath into its nostrils and the man began to live.
Then the Lord God planted a garden in Eden, in the East, and there he put the man he had formed. He made all kinds of beautiful trees grow there and produce good fruit.
Genesis 2:7-9, GNT
Azeroth as it (mostly) is today started with the Ordering thereof. During this period, the Titans began shaping the natural features of a then chaotic world. At the same time, they created three so-called "Seed Races" from living stone, to inhabit the world: earthen, vrykul and giant. As they were made of stone, they were (in the Warcraft meaning of the word) immortal. In shaping Azeroth, the Titans encountered the Old Gods, who were innately evil and symbiotically attached to this world. In destroying the Old Gods, the Titans would also destroy Azeroth, so they were forced to imprison them deep below the surface.
The "matrix" of nucleotide building blocks in DNA.
However, the Old Gods were able to propagate the Curse of Flesh, a "matrix destabilisation". In effect, it softened many of the Seed Races, shortening their lifespans dramatically and rendering them mortal. In creating the earthen, the Titans went through three iterations. The Series One experiments were a failure, not only because the resulting form was well short of ideal, it also succumbed to the Curse. This species is currently known as the troggs. The Series Two experiments produced the intended form (that of the dwarves), but the results were still susceptible. The Titans succeeded with their goals with Series Three, giving birth to earthen proper. The earthen were able to withstand the Curse, and stay stone-like to the present.
There was only one iteration of the vrykul, which succumbed to the Curse. They survive to this day.
At some time after the Pantheon (the ruling council of the Titans) left Azeroth, Vrykul mothers gave birth to children who were noticeably smaller and physically weaker than normal. King Ymiron (vrykul society's head of state) ordered them culled, but many mothers opted to abandon them instead. These children were shipped off to the Eastern Kingdoms (they were inhabiting Northrend), where they developed as humans.
Throughout Warcraft history, the Burning Legion (a demonic nation) have been trying to conquer Azeroth. In their third bid, their Supreme Commander Kil'jaeden converted the orc warlock Ner'zhul into the Lich King and tasked him with concocting a Plague of Undeath to (among others) be spread among the citizens of Lordaeron. Ner'zhul was successful, and was able to task Kel'thuzad with transporting it to the Eastern Kingdoms. There, Kel'thuzad formed the Cult of the Damned out of oppressed Lordaeron citizens and tasked them with propagating the Plague. It would kill the infected after a typical three days, then raise them as undead beings. These would do the bidding of the Lich King, as they had lost their free will in the raising process. The playable Forsaken undead regained their free will after a Nathrezim (dreadlord) raid on Lordaeron, which somewhat waned the hold of the Lich King over many of the undead.
The giants were created by the Aesir (storm giant) Titans to help with shaping Azeroth during the Ordering and when the Titans left. The direct creations were the mountain and sea giants, who were respectively tasked with shaping mountains and seas. From the mountain giants developed the frost giants. The giants have mostly resisted the effects from the Curse of Flesh, though Archavon the Stone Watcher reports having felt a soft face on himself.
The origins of the mechagnomes are currently unknown, though most have felt the destabilising effects of the Curse. These mechagnomes developed into the current gnomes. However, Gearmaster Mechazod found a way to cure affected gnomes of this Curse, effectively returning them to their robotic selves. Most gnomes would refuse this cure.
The Aliens
The eredar race originally lived on the planet Argus. However, Sargeras, a formerly Vanir (earth giant) Titan, was seeing a folly in the Pantheon's ways and started forming the Burning Legion to reverse their work. He offered the three eredar leaders (Archimonde, Kil'jaeden and Velen) great power in exchange for their loyalty. The former two accepted, and as a result much of eredar society transformed into the man'ari eredar, who joined the Burning Legion. Archie and KJ became high-ranking officers in it.
Velen, being a prophet, foresaw horrid visions of the Legion's actions, and as a result was inclined against joining. Naturally, for conflict-of-interest reasons, Velen and the uncorrupted eredar under his care were forced to flee Argus and was chased throughout the Twisting Nether, hopping worlds until they managed to find the secluded Draenor. There, they developed into the draenei and coexisted with the native orcs.
The Legion eventually found the draenei in Draenor. They introduced warlock magic to the orcs after appealing to them and their leader the then-shaman Ner'zhul. Orcs originally have brown skin, but exposure to such demonic magics will turn it green. This will occur even in orcs that do not practice such magic but are in regular proximity to practitioners.
The Mag'har section of orc society remained untouched by corrupting fel energy. They lived in Garadar, which was originally intended as isolation from the red pox outbreak pre-dating the arrival of the Legion. Nonetheless, it also isolated these orcs from practising warlocks, and their skin remains brown to the present.
The draenei fled again from Draenor, though some opted to stay behind. These draenei were particularly susceptible to demonic corruption, and many degenerated into the Broken and Lost Ones.
Some orcs have also been drinking the corrupting blood of the pit lord Magtheridon. over time, this turns them into the current fel orcs, characterised by great physical strength, red skin and all manner of protrusions.
The long-eared and bovine races
The Zandalari trolls existed since the beginning of the world, though their creation is unknown. As troll society expanded, trolls started inhabiting different climates, eventually adapting to their respective environments. The ice trolls living in Northrend developed pale blue skin and hardy hair. The sand trolls living in Tanaris developed golden-yellow, coarse skin. The skin of forest trolls living in Lordaeron is able to support plant life. The skin of jungle trolls (of which the playable Darkspear trolls are part) is covered with nice, soft fur (they would be very nice to hug!) which produces a purple appearance.
At the centre of pre-Sundering Azeroth lay a huge inland ocean called the Well of Eternity. It radiated arcane energy from the Twisting Nether and nourished all natural life. The night elves had developed on its shores, and there is a controversial theory that the Well's energies caused them to mutate from trolls. Nevertheless, the high elves are those night elves who utilised the Well's power in arcane magic. Abuse of such led to the Great Sundering, which tore the single continent into Kalimdor and the Eastern Kingdoms and destroyed the Well of Eternity. Arcane magic being addictive, the high elves developed peach-coloured skin. The playable blood elves are those high elves that, instead of fighting the addiction, chose to continue gorging themselves with arcane, demonic and holy satisfaction.
Some high elves were thrusted deep into the ocean during the Great Sundering. While some drowned, Queen Azshara secured an agreement with the Old Gods which allowed many to survive without air. These high elves became the naga.
The tauren and taunka are ancient races existing before recorded history. They are believed to have been created by the Earth Mother. However, there are theories equating the Earth Mother to the night elf god Elune, Therazane the Stonemother, Alexstrasza the Life-Binder (leading the Dragon Aspects), the Vanir Titan Eonar the Lifebinder and the Titans in general.