Melody "Mels" Pond, later River Song, was a "child of the TARDIS" 

She was human with Time Lord DNA, conceived by her parents, Amy Pond and Rory Williams, onboard the Eleventh Doctor's TARDIS as it travelled through the Time Vortex.

Melody was stolen from her parents as an infant by Madame Kovarian to become a weapon of the Silence in their war against the Doctor. After her second regeneration, she killed the Eleventh Doctor, but then broke her mental conditioning to give her remaining regenerations to revive his corpse.

The Doctor and her parents left her to make her own way in the universe. With no connection to her family or the Silence, River became an archaeologist, ostensibly to track the Doctor through time. She crossed the Doctor's path on several occasions, generally with the result that she was meeting him at progressively earlier points in his own time stream. Hoping to avoid temporal paradoxes, the Doctor gave her a diary to keep track of their meetings and to prevent her from revealing "spoilers" to him about his own future.

She died saving the Tenth Doctor, Donna Noble, Strackman Lux, and the 4,022 people saved in the computer system in the Library. The Tenth Doctor saved her consciousness digitally to the Library's computer system CAL, granting her a form of immortality.

Biography from the Tardis Wiki article, licensed under CC-BY-SA