Sara O'Neill was the wife of Jack O'Neill. After their son Charlie accidentally killed himself with Jack's gun, and Jack's subsequent trip through the Stargate, their marriage ended.
A little over a year after their divorce, Sara was approached by a man she thought was Jack, but who was, in fact, an alien duplicate. The alien told her that he/it was looking for Charlie and that he/it had to get back to the Stargate, which she had never heard of. When Sara saw the real Jack O'Neill she was very surprised. (SG1: "Cold Lazarus") She and Jack still meets sometimes as friends. She does know what the Stargate is.
Trivia
In the Stargate film, her name was Sarah O'Neil, and she had not yet divorced her husband.