Elf

History
There are several groups of elves in the Azeroth, but all are derived from one group: the Kaldorei, also known as night elves and its upper caste the Highborne.

Kaldorei, the night elves
"...a primitive tribe of nocturnal humanoids cautiously made their way to the edges of the mesmerizing, enchanted lake. The feral, nomadic humanoids, drawn by the Well's strange energies, built crude homes upon its tranquil shores.

Over time, the Well's cosmic power affected the strange tribe, making them strong, wise, and virtually immortal. The tribe adopted the name Kaldorei, which meant "children of the stars" in their native tongue."

Quel'dorei, the Highborne
The magocratic society that the night elves developed split into two castes: a body of the majority of the Kaldorei and the elite caste, the Highborne, known as the quel'dorei ("noble children", "highborne", or "high elves"). These were the favorites of and most loyal to the beautiful Queen Azshara, queen of the elves. They sought to understand the energies of the Well of Eternity, gaining exhilarating power from the arcane energy siphoned from the Well's depths.

The night elves (and in particular the quel'dorei) would go on to expand across ancient Kalimdor, shattering the empires of the Gurubashi and Amani trolls in the process and inspiring awe in those who witnessed their might. However, the arrogance and carelessness of the quel'dorei would spell doom for their civilization, attracting the attentions of the Burning Legion. Azshara and some of her Highborne opened a portal from the Well of Eternity into the Twisting Nether, enabling the demons to enter into Azeroth and wreak havoc upon Kalimdor. Some of the Highborne grew terrified of Queen Azshara's undying loyalty to Sargeras and the demons of the Burning Legion. Quickly, they escaped to aid the rest of the night elven civilization in banishing the demons back into the Twisting Nether in a catastrophic war now called the War of the Ancients, ending in the implosion of the Well of Eternity and the Sundering of Kalimdor.

Quel'dorei, the high elves
The Highborne were never fully trusted by the rest of the night elves who had abandoned magic to pursue druidism. The surviving night elves would end up banishing the Quel'dorei for the acts leading to the Sundering as well as more wanton waving of arcana. The quel'dorei were forced into exile and eventually settled in Quel'Thalas. Becoming known as "high elves," they took up a diurnal cycle instead of nocturnal and over time, their skin became a pale color instead of their former blue-green hues.

Sin'dorei, the blood elves
After the destruction of Quel'Thalas, the majority of the quel'dorei changed their nation to the sin'dorei, or "blood elves", in memorium of those who had fallen in the siege. Some, believing that their culture was eroding and were horrified at the new extreme measures taken by their prince, Kael'Thas, chose to continue calling themselves quel'dorei (high elves) and remained loyal to the Alliance. A notable difference between the high elves and blood elves is that, while the high elves combat their addiction to magic, the blood elves sucumb to it. The high elves spend at least one hour a day meditating, trying to restrain their addiction, while the blood elves drain the mana out of many creatures, including demons. This addiction sommetimes has grave consequences.

Shal'dorei, Nightborne, the dark elves
After cutting themselves off in their city behind a magical shield, this enclave of highborne became utterly dependent on their source of magic, the Nightwell. Over time this dependence and isolation changed them so their skin became a dark, gray-blue.

Elves that arrived through the Portals
Shortly after Vamir destroyed everything, two portals opened up into different areas and Civilization started being built around them.

Xen'Drik's Elves

Faerun's Elves