Stealth is a characters way of avoiding detection by sticking to shadows or using special clothing (like the ghillies suit) to break up the shape of the body and blend into the background. Other methods may be developed (via magic, science, etc..) to overcome secondary sensory systems like a radiant barrior to thwart thermal vision
This is a contested skill. Your stealth levels + focus + agility +1d20 vs ALL nearby characters awareness + spot levels +1d20 (per sensory system, including, hearing [use listen instead of spot] and scent related.)
The stealth user needs only one roll. He basically sets the difficulty for the opponents to become aware of his presence.
The DM may add a modifier to either side of roll depending on the environmental conditions. For example a stealth user may work well under a moonless night vs opponents with just normal vision and gain a +5 to the stealth users roll. By the same rule a stealth user under the same conditions but verses a race with thermal vision may stand out like a glowing sign granting the opponents a +5 to their rolls.