If people are observing you using any of their precise senses (typically sight), you can't use Stealth against them. (For more information on precise senses, see Perception.) Effects such as blur and displacement, which leave a clear visual of the you within the perceiving character's vision, aren't sufficient to use Stealth, but a shadowy area or a curtain work nicely. The hide in plain sight class ability allows a creature to use Stealth while being observed and thus avoids this whole situation.
You can move up to half your normal speed and use Stealth at no penalty. When moving at a speed greater than half but less than your normal speed, you take a --5 penalty. It's impossible to use Stealth while attacking, running, or charging.
If your observers are momentarily distracted (such as by a Bluff check), you can attempt to use Stealth if you can get to an unobserved place of some kind. This check, however, is made at a --10 penalty because you have to move fast.
Your Stealth check is opposed by the Perception check of anyone who might notice you. Creatures that fail to beat your Stealth check are either unaware of you or aware of your presence (see below) and treat you as if you had total concealment.
Breaking Stealth: When you start your turn using Stealth, you can leave cover or concealment and remain unobserved as long as you succeed at a Stealth check and end your turn in cover or concealment. Your Stealth immediately ends after you make an attack roll, whether or not the attack is successful (except when sniping, as noted below).
Sniping: If you've already successfully used Stealth at least 10 feet from your target, you can make one ranged attack and then immediately take a move action to use Stealth again. You take a --20 penalty on your Stealth check to maintain your obscured location.
EDITORS NOTE: There is additional information in Ultimate Intrigue and Giant Hunters Handbook