The mug skill allows thieves to begin an altercation on their own terms.
When mugging someone, you use the element of surprise in an attempt to murder your mark as quickly and as brutally as possible. Such a strike may leave your victim defenseless for a short period of time, further increasing the likelihood of selling their corpse on the black market.
Occasionally, however, a bit more discretion is required. Mugging with a light touch requires a bit more skill, attempting to knock out your victim without causing unnecessary harm. With your mark unconscious, robbing them becomes a simple task.