Sneaking makes it harder for your character to be detected. Using this skill is a toggle. You are either Sneaking, or you are not. When you are Sneaking, you don't actually know if the skill is working properly. When you first start Sneaking, and every minute after, the computer rolls against your Sneak skill. If successful, all critters will have their Perception halved when trying to notice you.

Normally, if your character runs, it will automatically turn Sneak off. You can tell if your character is trying to use this skill if the Sneak icon is displayed above the interface bar.