See also: Character Combat
A Helpless opponent- one who is bound, sleeping, unconscious, or otherwise at your mercy- is an easy target. You can sometimes approach a target that is unaware of your presence, get adjacent to it, and treat it as Helpless.
If the target is in combat or some other tense situation, and therefore in a state of acute awareness and readiness, or if the target can use its Dexterity bonus to improve its Reflex Defense, then that target can't be considered unaware.
Furthermore, any reasonable precautions taken by the target- including stationing bodyguards, placing its back to the wall, or being able to make Perception checks- also precludes catching that target unaware and Helpless.
Attacking a Helpless Opponent[]
A melee attack against a Helpless opponent gains a +5 bonus on the attack roll (equivalent to attacking a Prone target). A ranged attack gets no special bonus.
In addition, a Helpless opponent can't add its Dexterity bonus (if any) to its Reflex Defense. In fact, its Dexterity score is treated as if it were 0, so its Dexterity modifier to Reflex Defense is -5.