At the start of a battle, before you have had a chance to act (specifically, before your first regular turn in the Initiative order), you are Flat-Footed. You can't use your Dexterity bonus to AC(if any) while Flat-Footed. Barbarians and rogues of high enough level have the uncanny dodge extraordinary ability, which means that they cannot be caught Flat-Footed. Characters with uncanny dodge retain their Dexterity bonus to their AC and can make Attacks of Opportunity before they have acted in the first round of combat. A Flat-Footed character can't make Attacks of Opportunity, unless he has the Combat Reflexes feat.