When a combat starts, if a character was not aware of her enemies and they were aware of the character, that character is surprised. Likewise, a character can surprise her enemies if the character knows about them before they’re aware of the character.
The Surprise Round
If some but not all of the combatants are aware of their opponents, a surprise round happens before regular rounds begin. The combatants who are aware of the opponents can act in the surprise round, so they roll for initiative. If no one or everyone is surprised, a surprise round does not occur.
Combatants who are unaware at the start of battle do not get to act in the surprise round. Unaware combatants are still flat-footed because they have not acted yet. After the first turn of the round, if the aware combatants still have additional standard actions left (ergo the round hasn’t ended), the unaware combatants are no longer flat-footed but can only take immediate actions.