Combat Essentials:

This one page summarizes the basics of combat. If you're unfamiliar with the d20 rules, this is enough detail to get you started in a fight scene. For now, don't worry about the details.

Rounds:

Combat is broken up into rounds. Every round, each combatant gets to do something. A round represents about six seconds in the game world.

Before the first round of combat, each player makes an initiative check for his character. The GM makes initiave checks for the monsters or foes. An initiative check is a Dexterity check (1d20 + Dexterity modifier). Characters act in order from highest initiative result to lowest, with the check applying to all rounds of the combat.

A characther is flat-footed until he takes his first action.

Actions:

You can move and make a single attack in one round (in either order). A melee attack is one using a weapon you physically strike an opponent with, such as lunging with a knife or swinging a baseball bat. (A monster's claw attack is also a melee attack.) A ranged attack is one that you use at a distance, such as throwing a rock or firing a pistol.

Attack Roll:

To score a hit that deals damage on your attack roll, you must roll the target's Defense or better:

Melee Attack Roll: 1d20 + base attack bonus + Strength modifier + size modifier.
Ranged Attack Roll: 1d20 + base attack bonus + Dexterity modifier + size modifier.

Damage:

If you score a hit, roll damage and deduct it from the target's current hit points. Add your Strength modifier to damage from melee and thrown weapons.

If you're using a melee weapon in your off-hand, you add half your Strength bonus instead. If you're wielding a melee weapon with both hands, add one and one-half times your Strength bonus to the damage.

Defense:

A character's Defense is the number you need to get on your attack roll to hit that character in combat.

Defense: 10 + base defense bonus + Dexterity modifier + armour bonus + size modifier.

Hit Points:

Hit points represent how much damage a character can take before being disabled, knocked unconscious, or killed.

Attack Options:

When attacking, you have several basic options:

Attack: You can move up to your speed and make a single attack, or attack and then move.
Charge: When making a charge, you move in a straight line for up to double your speed and then make one attack with a +2 bonus on the attack roll. You suffer a -2 penalty to your Defense until your turn the following round.
Full Attack: Some characters can strike more than once each combat round, but doing so is a full-round action.

Movement:

Each character has a speed measured in feet. In one round, you can move that distance and attack. You can move before or after attacking.

In one round, you can run make a double move instead, running all-out. This lets you move double your speed but takes your entire turn.

Death, Dying, and Healing:

As mentioned above, your hit points represent how much damage you can take before being disabled, knocked unconscious, or killed.

Disabled: If your hit points drop to 0, you are disabled. You can only take one attack action or move action per turn. If you attampt any strenuous activity, you take 1 point of damage after completting the act. You may continue to act as if disabled until your negative hit point total exceeds your Constitution bonus. For example, if your Con modifier is +3, you are disabled from 0 to -3 hit points.
Dying: If your negative hit point total is greater than your Constitution modifier but still less than your Massive Damage Threshold, you're unconscious and dying. You lose 1 hit point per round. Each round, before losing that hit point, you have a chance to stabilize equal to your Constitution score. While stabilized, you're still unconscious. Each hour, you have a chance to regain consciousness equal to your Constitution score; if you don't you lose 1 hit point instead.
Dead: If your negative hit point total exceeds his Massive Damage Threshold, you're dead.

You can stop a dying character's loss of hit points with a successful Treat Injury check (DC 15).

Massive Damage:

If you sustain damage from a single attack that exceeds your Massive Damage Threshold (and the loss of hit points doesn't kill you outright!) you must still make a Fortitude save (DC 15 + 1 per 5 points of damage). If this saving throw fails, you immediately drop to 0 hit points and are disabled.