Guinan was a time sensitive native to a parallel universe. She worked as a bartender in the 24th century in the USS Enterprise-D's lounge and recreation facility known as Ten Forward.
In 2366, she began receiving memories of things that she felt she should not actually remember. When the Eleventh Doctor arrived on the Enterprise and experienced the same problem in 2368, Captain Jean-Luc Picard brought him to Ten Forward to meet Guinan, and they found that they had a familiarity with each other that both knew they shouldn't otherwise have, which they recognised as due to their shared ability to sense disturbances in time. They also realised that the Cybermen had somehow altered history in order to bring their respective universes together and forge an alliance with the Borg. (COMIC: Assimilation²)
Edit