The Xylok were a mysterious crystalline race.
The Xylok seemed to be made of clear crystals, but they had a very unusual structure and were actually alive. They could contact other Xylok from afar and interface with computers. They could also apparently survive for millions of years. Due to their crystalline and computer-like nature, they were also vulnerable to 21st century Earth computer viruses. (TV: The Lost Boy)
In roughly 60 million B.C., a Xylok meteorite crashed into the planet Earth. All of the Xyloks within were trapped in Earth's crust. In 1883, during the volcanic eruption at Krakatoa, one Xylok crystal was released from inside the Earth. This crystal was eventually picked up by geologists, who could not classify it and sent it to Sarah Jane Smith. The Xylok communicated with her via her laptop and convinced her that he could help her defend the Earth. It designed an advanced computer system which she kept hidden in her attic. The Xylok, called Mr Smith, used this computer as a body and bided its time.
It contacted the Slitheen and convinced them to help him. Though they believed he was helping them collect telekinetic power, he was actually manoeuvring events so he could use the telekinetic powers of Luke Smith to crash the Moon into the Earth. This would rip apart the planet, releasing the Xylok. Sarah Jane infected Mr Smith with the Armageddon virus, which wiped his memories and stopped him from destroying the Earth. (TV: The Lost Boy)
Mr Smith returned to his original function. He was integral, in combined efforts with Torchwood Three and Martha Jones, in contacting the Tenth Doctor when Daleks invaded Earth. He was then able to use the TARDIS base code to help transport the Earth back to its original position in space. (TV: The Stolen Earth/Journey's End) Afterwards, Mr Smith continued his efforts to protect Earth, which Sarah Jane declared to be his new purpose after erasing his memories. (TV: The Lost Boy, et al.)