Guinan is an El-Aurian. Her people, sometimes called "listeners," were scattered through the galaxy when the Borg invaded their homeworld. As a refugee aboard the El-Aurian vessel Likul, she was rescued from the Nexus by the Enterprise-B (Star Trek: Generations).
Her species is long lived, and they apperently have some powers beyond what is associated with humanoids. Q seemed to treat her as an equal in his encounters with her. In the episode "Yesterday's Enterprise," she was able to sense that the reality they ended up in was not the proper flow of time, even though everyone else believed it to be the natural course of events.
The character was named for famed saloonkeeper Texas Guinan.
Guinan was played by Whoopi Goldberg.