GLaDOS

GLaDOS stands for Genetic Lifeform and Disk Operating System.  She is a sentient computer.  It was on the day that she was first activated (which happened to coinside with the first Aperture Science bring your daughter to work day) that GLaDOS became self aware and locked down the facility.  The survivors trapped inside the Aperture Science facility made GLaDOS angry and she began to flood the enrichment center with a deadly neurotoxin.  The survivors fitted GLaDOS with a Morality core which quelled her rage.  Glados continued to test the survivors until all that was left was Chell. 

GLaDOS begins to show instability as you travel throughout the game.  She slowly becomes more and more emotional.  When Chell outsmarts GLaDOS by jumping off the platform before she gets burned alive that’s when GLaDOS’s true nature begins to be truly revealed. 

At the end of the game though it may seem that GLaDOS is destroyed it is really only a part of her that is destroyed.  It is there that the song Still Alive picks up.