Dark Elves are a faction of elves that use a special race icon to differentiate them from their mainstream brethren. The difference between Dark Elves and common Elves is cultural and political, and this split developed sometime during the Age of Legends. Like ogres and orcs, they are unplayable as a separate race. But if playing as an elf, then the player can use the character background of Dark Elf Follower to basically play as one.
Dark Elves are actually the same race as elves, and they are physically identical and indistinguishable from regular elves. And that is a major plot point in the evil path, where dark elves have secretly infiltrated and taken over the Panarii church, posing as regular elves.
Dark Elves are quite racist and xenophobic, seeing non-elven races as being too incompetent to govern themselves. They tolerate other elves and half-elves. Most of them admire Arronax. They are the subject of discussion in the book T'sen-Ang: Horror Among the Dark Elves.
Dark Elves tend to live in T'sen-Ang, but are occasionally encountered as NPCs, especially in the larger cities like Tarant and Caladon.
Some Notable Dark Elves can be found in the Elf article.