A character's Attack Bonus is a derived stat that determines how likely that character is to successfully hit an opponent in combat, and is opposed by the target's Armor Class. When attacking, you roll a d20 and add your Attack Bonus to the roll. This is your Attack Roll, and if the result equals or exceeds the target's Armor Class, you hit the target. If you hit with a weapon, you roll damage and subtract the result from the target's Hit Points (assuming their Damage Reduction doesn't negate your damage). If you hit with a spell (usually by performing a Touch Attack or Ranged Touch Attack), you may then need to overcome their Spell Resistance, a Saving Throw or elemental resistances before applying damage or whatever other effects the spell may have.
One of the primary factors determining your Attack Bonus is your Base Attack Bonus, a score that increases at varying rates depending on what classes you pick and ultimately determined by a combination of your class choice(s) and character level (see Base Attack Bonus).
Attack Bonuses can be fairly complicated and situational, however. In addition to your Base Attack Bonus, other contributing factors include weapon choice, feats, bonuses from magical weapons and items, the use of various abilities, ability score modifiers and circumstance bonuses (flanking, invisibility, etc.) and spell buffs.
Melee weapons tend to add (or subtract) a character's Strength Modifier, while ranged weapons use a character's Dexterity Modifier. Some light melee weapons can, with the Weapon Finesse feat, allow a character to use their Dexterity Modifier to influence their Attack Bonus - a wise choice for, say, dagger-wielding rogues.
You equipment also plays a significant role in determining your Attack Bonus. Some varieties of equipment or fighting styles may actually lower your Attack Bonus (dual wielding, using a Tower Shield, fighting with an oversized weapon or a weapon you're not proficient with). Generally, however, you'll be dealing with bonuses, usually in the form of masterwork or enchanted weapons, which provide a +1 to +5 bonus to your Attack Bonus. Other gear may also increase your Attack Bonus, generally with specific weapons (Bracers of Archery) or against a type of enemy, and of course ability-boosting gear (Belt of Giant Strength) may increase your ability scores, in turn further increasing your Attack Bonus.
Below you'll find a list of conditions that may affect a character's Attack Roll:
|Attacker's Condition||Attack Modifier|
|Firing ranged weapon into combat||-4|
Performing a single attack can be done as a Standard Action, while making multiple attacks (for characters with a sufficiently high Base Attack Bonus) requires a Full-Round Action.