Mr Smith was a super-computer (of sorts) owned by Sarah Jane Smith. The computer was located in the attic of Sarah Jane's house at 13 Bannerman Road.
Biography
Origins
The computer known as Mr Smith housed a sentient and highly intelligent crystal known as a Xylok. In the distant past a Xylok ship crashed into the Earth, trapping the Xyloks the crust of the planet. When the volcano Krakatoa erupted, one crystal escaped. Decades later, Sarah Jane Smith retrieved the living crystal, Sarah Jane took in her home on 13 Bannerman Road and found it able to communicated with her via her laptop computer. It built itself a body out of machines. This way, the crystal informed her as to how to build the computer which would house the living crystal. Sarah Jane did not know at this time about the Xyloks or that Mr Smith ultimately planned to free the other Xylok, even at the cost of the planet itself. (SJA: The Lost Boy)
Reprogramming
Mr Smith teamed up with the remaining Slitheen on Earth, and, using deception, helped two of of them to pose as Luke Smith's true parents so that they could kidnap him. Sarah Jane used a computer virus against Mr Smith which disabled him and caused him progressive amnesia. Using this to her advantage, Sarah Jane informed him of his mission to serve humanity. (SJA: The Lost Boy)
Abilities
Mr Smith possessed knowledge of and kept a database on many alien races, and secretly monitored the activity and communications of alien ships.