Sneak is a Rogue skill used to attempt to enter a room unseen. The chance of success of a sneak attempt depends on the percent to which it is practiced, as well as (possibly?) the amount of Notice the room inhabitants have. While important for setting up a Backstab, it plays no part in the actual landing rate or damage inflicted.
In Game Description
Syntax: sneak Thief Skill: This causes you to attempt to move silently, masking your movements from those around you. This skill can be very useful in attempting to steal from people and is a requirement for attempting to backstab someone. Success depends upon how well you have trained in this skill. Note that sneaking takes extra movement points and will tire you quickly if left on. Sneak again to toggle it off.