Stealth is used whenever a character wants to do something without being noticed, or something that requires a combination of knowledge and dexterity. In game mechanic terms, it is a combination of agility and intelligence.
Whenever someone might notice something sneaky going on, they make a perception check. Perception is a combination of intelligence and wisdom. As always, the highest total wins. So, if the thief rolls higher, then he continues on, unnoticed. However, if the observer rolls higher, he catches the thief in the act (and probably calls for the guard). Ties are awarded to the observer, although players may spend a karma point to win the contest, instead.
The environment plays a big role in the success or failure of stealth attempts. Anything that makes it more difficult to see or hear gives a bonus to stealth, and anything that makes it easier to see or hear gives a bonus to perception.
Very bright light: +4 bonus to perception rolls.
Very dim light: +4 bonus to stealth rolls.
Very quiet environment: +2 bonus to perception rolls.
Very noisy environment: +2 bonus to stealth rolls.
Activity/distractions in the environment: +2 bonus to stealth rolls
Looking directly at the thief: +10 bonus to perception rolls.