At the beginning of a battle, every combatant makes an initiative check. An initiative check is a Dexterity check. Each character applies other modifiers from spells, feats, and results, in addition to their Dexterity modifier to the roll. Characters act in order, counting down from the result to the lowest. In each round that follows, the characters behave in precisely the exact same order .
Each round's action begins with the character with the highest initiative outcome and then proceeds in order. That character performs his entire round's worth of actions, when a character's turn comes up in the initiative series.
If some but not all the combatants are aware of their opponents, a surprise round happens before regular rounds begin. In initiative order , combatants who started the battle aware of their opponents each take a regular or move action during the surprise round. You can take actions that are free . No surprise round occurs, if everyone or no one is surprised.
Once the rules refer to a 'full round', they usually indicate a period of time from a particular initiative count in one round to the same initiative count in the next round. Effects that last a certain number of rounds end before the same initiative count that they began on.
Sometimes all the combatants on a side are aware of their opponents, sometimes none are, and sometimes only a few of them are. Occasionally a few combatants on each side are aware and the combatants on each side are unaware.