The Data Core was a huge hard drive at the centre of the Library. Felman Lux used it to provide a virtual reality for Charlotte to thrive in. An artificial moon called Doctor Moon transmitted waves to and from the Core to keep Charlotte happy, and a father was created for her.
The people in the library were saved into the Core, in cyberspace, after there was nowhere for them to be safely teleported to, and Doctor Moon also helped them. Donna Noble later got caught up in Doctor Moon and the Core's transmissions when she was being teleported, and she began to live in the Core, marrying Lee McAvoy and having two children. The data ghost of Evangelista had also been saved; however, her data was corrupted, resulting in her becoming vastly intelligent but horribly deformed. River Song freed everyone from the Core by sacrificing her life to add her memory space to the Data Core. Evangelista was restored to her original appearance, and the data ghosts of Proper Dave, Other Dave and Anita were saved. The Tenth Doctor saved River's data ghost into the Core. (TV: Forest of the Dead)
After this, the Data Core contained River, Charlotte, Evangelista, Anita and the two Daves. (TV: Forest of the Dead) River returned from the Data Core as an echo to help the Eleventh Doctor one last time as he went to Trenzalore by guiding Clara Oswald through a psychic "conference call". (TV: The Name of the Doctor)