GLaDOS is the massive supercomputer of Apeture Science. GLaDOS is one of the only 2 main characters in the first Portal, and is still a main character in Portal 2. GLaDOS had different cores plugged into the sides of her. The last core was plugged in late, but this was because she flooded the Enrichment Centre with a deadly nerotoxin, the core was the Morality core (Followed on from her curisoty, anger and inteligence cores) and was supposed to stop her from flooding the enrichment centre. When Chell meets GLaDOS, GLaDOS mentions a suprise, but instead of the suprise (most likely going to be nerotoxin) her morality core falls out. Trying to tell Chell to leave it alone, Chell incinerates it, and she starts flooding the enrichment centre again. GLaDOS stands for Genetic Lifeform and Disk Operating System. GLaDOS also was killed by Chell, but re-awakened by Wheatly and Chell a later few thousand years on, and want to continue testing.
GLaDOS's Morality core causes her to lie.
GLaDOS: You are doing, very well!
GLaDOS: You will be baked, and then, there will be cake.