Bards use battle cries to encourage, inspire, warn, and magically aid their allies. Battle cries are triggered by flexible melee attacks. The bard makes a melee attack and is able to use a battle cry that corresponds to the attack's natural result, sometimes whether or not the attack hits.
Bonuses provided by battle cries can help a bard's allies but not the bard.
You can generate the effect of any 1st or 3rd level battle cry you know as a standard action (instead of making a flexible melee attack to see which battle cry you are able to trigger). This allows you to choose the particular battle cry you want, at the expense of taking your standard action.