A sorceress is the female version of a sorcerer.
As with the elven priests, the elven sorceresses who remained in Lordaeron paid little heed to their race’s departure from the Alliance. These female magic users, serving as agents to the Kirin Tor of Dalaran, use their arcane powers to aid the Alliance in times of peril.
These female magi, sent by the magocrats of Dalaran to watch over the volatile battle magi, use their arcane powers to aid the Alliance forces whenever Humanity is imperiled. Although their powers are not often used directly in combat, the sorceresses are able to aid the Alliance warriors with a wide array of specialized conjurings and magical effects.
Note that both high elf and blood elves sorceress have the symbol of Dalaran on their cloaks.
High elven sorceresses
The Sorceress helped the high elves fend off Arthas as he tried to attack their capital Silvermoon and Quel'Thalas in Warcraft III. It is a caster unit. It is produced at the high elf building called the high elf arcane sanctum. The blood elves also had sorceresses in The Frozen Throne expansion campaign in Warcraft III. Their damage and armor type are different than the expansion sorceress.
Blood elf sorceresses
Sorceresses are a blood elf unit that is a caster. They appeared in the Warcraft III: The Frozen Throne human campaign to aid Kael'thas. It is produced at the Blood Elf Arcane Sanctum.