You mean River Song? We don't know much so far. She was imprisoned in the Stormcage Holding Facility for killing, "A very good man. Best man I've ever known." Apparently each time the Doctor meets her, it's earlier in her timeline and later in The Doctor's. So in their first meeting in Silence of the Library she knew him very well, and he met her for the first time. She's shown extreme knowledge and familiarity with the TARDIS, piloting it better than The Doctor does. Each time The Doctor meets her, she references their next adventure, indicating that she does in fact know The Doctor's future.
In the season 6 episode 7, A Good Man Goes To War, we learn that River is
Amy and Rory's child. Conceived in the TARDIS as it was traversing the Time Vortex, the baby absorbed some vortex energy, becoming part Time Lord. She was abducted as a baby by Madame Kovarian and The Church. At some point after that, she ended up on Earth as a young child, where The Silence cared for her. They built a spacesuit for her that would sustain and protect her. Somehow River escaped from the suit and as a child, was last seen starving to death, and then regenerating in a city on Earth.
Much more detail can be found at the Doctor Who wiki.