The elven family tree

The trunk: the original Kaldorei

Ten thousand years ago, there was a single elven race: the Kaldorei (Night Elves). They lived near the Well of Eternity, in the heart of Kalimdor, and their power came from this source of pure magic.

The Kaldorei were an advanced but divided society. While most lived in harmony with nature, an intellectual elite — the Highborne — delved deeply into the arcane magic of the Well of Eternity, becoming addicted to it.

This division would cost everything.

The first branch: Quel'dorei (High Elves)

When the Legion was defeated in the War of the Ancients and the Well of Eternity was destroyed, the Kaldorei retreated to Hyjal. The Highborne were banished for having collaborated (even indirectly) with the invasion.

A group of these banished Highborne, led by Dath'Remar Sunstrider, sailed east and founded Quel'Thalas—a new magical realm built around a source of power of their own creation: the Sunwell.

These are the Quel'dorei — the High Elves. Over the millennia, separated from the Kaldorei, they became smaller, lost their bluish skin and glowing eyes, and adapted to the temperate climate of the north.

The second branch: Sin'dorei (Blood Elves)

The Fall of Quel'Thalas during the Third War was the event that split the High Elves in two.

Arthas and the Scourge destroyed the Sunwell. With their source of power corrupted and destroyed, the High Elves went into magical withdrawal—physically painful, psychologically devastating. The majority of the population died.

Prince Kael'thas Sunstrider gathered the survivors and renamed them Sin'dorei — "children of blood," in honor of the dead. They would no longer be elegant High Elves: they would be a people defined by loss and the desperate search for a new source of magic.

These are the Blood Elves of the Horde in WotLK.

Those who stayed: Quel'dorei of the Alliance

Not all High Elves followed Kael'thas or joined the Horde. A smaller group rejected the new name and identity, remaining faithful to the Alliance.

They continue to be called High Elves or Quel'dorei. In WotLK, they appear mainly through the Silver Covenant in Dalaran — led by Vereesa Windrunner, sister of Sylvanas.

They are the elven minority who chose continuity over reinvention.

The Forgotten Branch: Shen'dralar

The Shen'dralar are the Highborne who stayed in Kalimdor instead of heading east. They isolated themselves in Dire Maul, preserving arcane knowledge for millennia.

In WotLK lore they exist as collectors of books and secrets — technically close relatives of the High Elves, but who have taken a completely different path.

The Nightborne: extreme isolation

The Nightborne (Nightborne Elves) are a branch that few know well. They are the descendants of the Highborne who stayed in Suramar and isolated themselves for ten thousand years behind a magical barrier, supported by the Nightwell.

Ten thousand years of isolation has changed them completely—physically and culturally. When the barrier fell on Legion, they had to choose between Alliance and Horde. They chose the Horde.

The branch of the Void: Void Elves

The Void Elves are Blood Elves who were banished from Quel'Thalas for experimenting with Void magic — prohibited by the Sin'dorei. Alleria Windrunner found them and trained them to control the power without being consumed by it.

They joined the Alliance. They are, genetically, Blood Elves — but choices and power separated them completely.

The summary tree

Kaldorei (Night Elves)
└── Exiled Highborne
    └── Quel'dorei (High Elves)
        ├── Sin'dorei (Blood Elves) — after the Fall of Quel'Thalas
        │   └── Void Elves — exiled for Void magic
        └── Alliance Quel'dorei — refused the new name

Highborne who stayed in Azeroth
├── Shen'dralar — isolated in Dire Maul
└── Nightborne — isolated in Suramar for 10,000 years

All of these races started at the same point. What separated them was a combination of choices, tragedies and millennia of different history.