When using hide in shadows or an invisibility effect, you will have to pass stealth checks to ensure that those around you do not detect or see you. Stealth checks are an opposed check between stealther and perceiver and work as follows:

Stealth + 10 VS Perception + d20

When hiding, stealth equals stealth skill. In addition, if using magical invisibility, the spellcraft skill is also added to your stealth UNLESS the perciever is using stealth nullification (true seeing, detect invisibility). This total is checked against the total perception skill of the perceiver.

The spot feat will enable a pulsing passive perception which can alert the perceiver of the presence of the stealther. This forces a stealth check on the stealther every round.

When using the search command, the perceiver forces an additional stealth check on the stealther with a -10 penalty.

Wearing armor gives you a penalty to your stealth check equal to 10 - your armor's max dexterity.

Size matters. Medium creatures get no bonus or penalty to stealth. Large creatures get a -4 penalty to stealth checks. Small creatures get a +4 bonus to stealth checks.