Hide

Hide (Dex;Armor Check Penalty)
The Hide skill is used once the Sneak feat is activated. The symbol of an eye appears besides your character to reflect your efforts. One eye means that you are hidden very well while with three or four eyes means that you're visible for almost everyone to see. Moving near walls and dark areas improves your Hide check.

A hidden character can be detected with a successful Spot check.

Note that when sneaking your Move Silently skill is of great importance, too.