Soon after, Kirito plays another game called Gun Gale Online (GGO) to investigate the mysterious connection between it and deaths occurring in the real world. Assisted by a female player he meets in the game called Sinon, he identifies and exposes the culprits, who include some former members of a murderous guild he had previously encountered in SAO.
Kirito is later recruited to assist in testing a state-of-the-art FullDive machine, Soul Translator (STL), which has an interface that is far more realistic and complex than the previous machine he had played. He tests the STL by entering a Virtual Reality world created with The Seed package, named UnderWorld (UW).
In the UW mainframe, the flow of time proceeds a thousand times faster than in the real world, and Kirito's memories of what happens inside are restricted. However, he ends up falling into a trap set by one of the murderers from GGO and wakes up inside UnderWorld without any recollection of how he ended up there, unable to log out, with his real self left in a comatose state.
Source: http://en.wikipedia.org/wiki/Sword_Art_Online