You know how to tell a lie.
Bluff is an opposed skill check against your opponent’s Sense Motive skill.
You can use Bluff to pass hidden messages to another character without others understanding your true meaning. The DC of this check is 15 for simple messages and 20 for complex messages. If you are successful, the target automatically understands you, assuming you are speaking in a language that it understands. If your check fails by 5 or more, you deliver the wrong message. Other creatures that hear the message can decipher the message by succeeding at an opposed Sense Motive check against your Bluff result.
Action: Delivering a secret message generally takes twice as long as the message would otherwise take to relay.
Retry? Yes. Secret messages can be relayed again if the first attempt fails.
If you use Bluff to fool someone, with a successful check you convince your opponent that what you are saying is true. Bluff checks are modified depending upon the believability of the lie. The following modifiers are applied to the roll of the creature attempting to tell the lie. Note that some lies are so improbable that it is impossible to convince anyone that they are true (subject to GM discretion).
Retry? If you fail to deceive someone, any further checks made to deceive them are made at a –10 penalty and may be impossible (GM discretion).
Source: Giant Hunter's Handbook
It's often useful to attempt to convince your enemies you are no threat to them.
Check: You attempt to convince your target you are harmless through your actions and posture. If you are at least one size category smaller than your target and have taken no effective offensive actions that your target has seen, you gain a +5 circumstance bonus on this check. Even if you have proven yourself capable of dealing damage, an effort to present your previous success as a one-time fluke takes only a –10 penalty on the check.
Retry? You can attempt to feign harmlessness to the same target again, but each previous failed check increases the DC to convince your target by 5. This increase resets after 1 hour has passed.
You can also use Bluff to feint in combat, causing your opponent to be denied his Dexterity bonus to his AC against your next attack. The DC of this check is equal to 10 + your opponent’s base attack bonus + your opponent’s Wisdom modifier. If your opponent is trained in Sense Motive, the DC is instead equal to 10 + your opponent’s Sense Motive bonus, if higher. For more information on feinting in combat, see Combat.
Action: Feinting in combat is a standard action.
Retry? Yes. You can attempt to feint against someone again if you fail.
Source: Giant Hunter's Handbook
You can use Bluff and Diplomacy together to make a request of a creature, without it even realizing you have made the request.
Check: You can gradually coax a target into thinking a suggestion is entirely its own idea, making the creature more likely to act on the idea than if you had suggested it outright. You discuss topics subtly relevant to the request, asking leading questions and narrowing the scope of the conversation so that the target eventually decides to take a specific action you have led it to.
You first attempt a Bluff check to convince the target that your request was actually its idea. This is always treated as far-fetched circumstances, resulting in a –10 penalty on the check. If successful, you then attempt a Diplomacy check to make the request of the creature, treating its attitude toward you as indifferent for this single request (regardless of its actual attitude).
Action: Planting the notion and then coaxing a target into suggesting the notion himself each require at least 1 minute of continuous interaction. This can be difficult to arrange with a hostile or unfriendly creature.