Ready Action

The ready action lets you prepare to take an action later, after your turn is over but before your next one has begun. Readying is a Standard Action. It does not provoke an Attack of Opportunity (though the action that you ready might do so).
You can ready a Standard Action, a Move Action, a Swift Action, or a Free Action. To do so, specify the action you will take and the conditions under which you will take it. Then, anytime before your next action, you may take the readied action in response to that condition. The action occurs just before the action that triggers it.
You can take a 5 Foot Step as part of your readied action, but only if you don’t otherwise move any distance during the round.
If the triggered action is part of another character’s activities, you interrupt the other character. Assuming they are still capable of doing so, they continue their actions once you complete your readied action.
Your Initiative result changes. For the rest of the encounter, your new Initiative result is the count on which you took the readied action, and you act immediately ahead of the character whose action triggered your readied action (if applicable).
If you come to your next action and have not yet performed your readied action, you don’t get to take the readied action (though you can ready the same action again). If you take your readied action in the next round, before your regular turn comes up, your Initiative count rises to that new point in the order of battle, and that becomes your turn for the new round.

Distracting Spellcasters

You can ready an attack against a spellcaster with the trigger “if they start casting a spell.” If you damage the spellcaster, they may lose the spell they were trying to cast (as determined by their Concentration check result).

Readying to Counterspell

You may ready a Counterspell against a spellcaster (often with the trigger “if they start casting a spell”). In this case, when the spellcaster starts a spell, you get a chance to identify it with a Spellcraft check (DC 15 + Spell Level). If you do, and if you have the ability to cast a spell that can counter it, you can do so. Counterspelling works even if one spell is divine and the other arcane. Detailed rules on how to counter spells can be found in the Counterspell article.

Readying a Weapon against a Charge

You can ready weapons with the brace feature, setting them to receive a Charge. A readied weapon of this type deals double damage if you score a hit with it against a charging character.