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The combat system is, unfortunately, fairly complex. However, I hope that I will be able to drastically simplify the explanation of the combat system for forum use.

(please feel free to skip the glossary. It is here for CONVENIENCE, and should be used primarily when someone bring sup a game term you don't understand)

Glossary of Terms:
Ability Damage: A temporary loss of ability score points. Lost points recover at a rate of 1 point per day per ability unless noted otherwise. Any ability score of 0 renders the character helpless, and a constitution score of 0 kills the character.
Ability Drain: a permanent loss of ability score points.
AC: Armor Class
Attack of Opportunity: An extra melee attack made during a round in which an adjacent opponent makes his/herself particularly vulnerable.
Automatic Hit: A roll of 20 on an attack roll is an automatic hit.
Automatic Miss: A roll of 1 on an attack roll is an automatic miss.
Blinded: Unable to see. A blinded character takes -2 to AC, loses dexterity bonus to AC, and takes -4 penalty on search checks and many strength and dexterity based skills. Character is unable to use Spot and to read. Opponents have 50% miss chance to avoid characters attacks.
Concealment: something that prevents the attacker from clearly seeing his target. Concealment causes a percentage chance that a successful attack may miss.
Coup de grace: A full-round action which allows the attacker to attempt a killing blow (automatic critical hit) against an adjacent, helpless opponent. The victim must succeed on a fortitude save against DC 10+damage taken or die.
Cover: Any barrier between attacker and defender
Cowering: Frozen in fear and unable to take actions, takes a -2 penalty to AC, and loses dexterity bonus to AC.
Critical hit: A hit that strikes a vital area and therefore deals extra damage appropriate to the weapon. The attacker must roll a critical threat (a "natural" roll that is within the weapons "threat range" on the d20), and then succeed on an attack with the same attack bonus to confirm the critical hit. A failed critical hit still hits normally.
Damage Reduction: A special defense which allows a creature to ignore a portion of the damage receives. Some damage reduction can be overcome with special weapons (such as silver or cold iron).
Dazed: Unable to act, but receives no penalty to AC.
Dazzled: Unable to see well due to over-stimulation to the eyes. -1 to attack rolls, spot and search checks.
Dead: A character dies when his/her hit points are equal to -10.
Deafened: Unable to hear. A deafened character takes -4 on initiative, automatically fails all listen c hecks, and has a 20% chance of failing all spells with verbal components.
Disabled: At exactly 0 hit points, or in negative hit points but stable and conscious. May take a single move action or standard action each round, moves at half speed. If the character takes a full round action, she receives 1 point of damage and falls unconscious.
Dying: unconscious and near death (-1 to -9 hit points). Each round, the character has a 10 chance to stabilize, or else receives 1 point of damage.
Entangled: Moves at half speed, -2 on attack rolls, -4 penalty to effective dexterity score. DC 15+spell level concentration check to cast spells.
Ethereal: On the ethereal plane. An ethereal creature is invisible and intangible to creatures on the material plane. Ethereal creatures can move in all directions, and through objects, but move at half speed.
Exhausted: Tired to the point of great impairment. -6 penalty to effective strength and dexterity. After 1 hour of complete rest, an exhausted character is fatigued.
Fascinated: Entr5anced in a supernatural or spell effect. A fascinated creature stands or sits quietly, taking no actions other than paying attention to the fascinating effect. -4 on all skill checks made as reactions (listen, spot, etc.). Any potential threat allows another saving throw against the effect.
Fatigued: Tired to the point of impairment. -2 to effective strength and dexterity.
Free Action: Free actions consume a negligible amount of time to perform (such as speaking a short message).
Frightened: fearful of a creature, situation, or object. A frightened creature flees from the source of its fear as best it can. If unable to flee, it may fight. A frightened creature takes a -2 penalty on attack rolls, saving throws, skill checks, and ability checks.
Full-round action: A full round action consumes all of a character's efforts for a full round (except for a 5-foot step (usually), free actions, and a single swift action). If a full round action is used to run, the character moves 4 times his/her speed, and cannot take a 5-foot step.
Hardness: The natural DR of an item.
Incorporeal: Having no physical body. They can be harmed only by other incorporeal beings, magical weapons (which suffer a 50% miss chance), or by ghost touch weapons.
Invisible: visually undetectable. An invisible creature gains +2 on attack rolls against sighted opponents, and ignores opponents dexterity bonuses to AC. Invisible creatures have a 50% miss chance to not be hit in combat. Invisibility provides +40 to hide checks while immobile, and +20 while moving.
Lethal Damage: Damage which reduces a creature's hit points normally.
Line of Effect: Determines whether or not an effect (like an explosion) can rach the target.
Line of Sight: An imaginary line between the source and the target which allows the source to see the target.
Move Action: An action that is the equivalent of the character moving his/her speed. Move actions include drawing weapons, standing up from prone, loading a light crossbow, moving your speed, etc. A character taking a move action may still take a standard action in the same turn.
"Natural" Roll: The actual number on the face of a rolled die.
Nauseated: Experience stomach distress. Unable to attack, cast spells, concentrate on spells, or do anything else requiring that sort of attention.
Nonlethal damage: Damage intended to subdue the target. If the character falls below 0 hit points due to nonlethal damage, they fall unconscious.
Panicked: A panicked creature drops anything it holds and flees. It can't take any action other than running. It takes -2 penalty on saving throws, skill checks, and ability checks. If cornered, a panicked creature cowers and does not attack, and typically takes the full defense action.
Paralyzed: Frozen in place, and unable to move or act. Has effective dexterity and strength scores of 0, and is helpless, but can take purely mental actions.
Petrified: Turned to stone.
Prone: Laying on the ground
Range Increment: The steps at which a ranged weapon takes penalties for range. Ranged weapons can reach up to 5 range increments, while thrown weapons can reach up to 10 range increments.
Range Penalty: Each range increment beyond the first causes a -2 penalty to attack rolls with the weapon.
Reaction: Automatic action in response to another action. For example, the DM may call for spot and/or listen checks as a reaction.
Shaken: Mildly fearful. -2 penalty on attack rolls, saves, skill checks, and ability checks.
Sickened: Mildly ill. A sickened character takes a -2 penalty on all attack rolls, weapon damage rolls, saving throws, skill checks, and ability checks.
Stable: unconscious and having negative hit points (from -1 to -9)
Staggered: Having nonlethal damage equal to current hit points. A staggered character may only take a single move action or standard action each round.
Stunned: A stunned creature drops anything held, can't take actions, takes -2 penalty to AC, and loses his dex bonus (if any) to AC.
Surprise: A condition at the beginning of battle if some (but not all) combatants are unaware of their opponents.
Temporary Hit Points: Temporary hit points are lost before regular hit points are.
Unconscious: Knocked out and helpless. An unconscious character who is at negative hit points has a 10% chance each hour to become conscious. A character unconscious due to subdual damage has a 10% chance each minute to wake up and be staggered until healed.

Movement
This section details the finer points of moving your character during combat. Please keep in mind that conventional Dungeons and Dragons is meant to move characters on a grid using 5 ft squares.

Types of Movement
5-Foot Step (free action)
Single move (move action)
Partial Charge (standard action)
Double Move (two move actions)
Charge (full round action)
Triple Move (full round action)
Run (full round action)

5-foot step
A 5-foot step moves the character one square in any direction. 5-foot steps do not provoke attacks of opportunity, unless you are moving into an opponents square without making a tumble check.

Charge and Partial Charge
If you charge or partial, you must move at least 10 feet. You gain a 2 +2 bonus to your attack roll, but suffer a -2 penalty to AC until your next turn. If you charge, you may move up to double your speed and attack as a full round action. If you partial charge, you may move your speed and attack as a standard action.

Double Move, Triple Move, and Run
Double move allows you to move up to double your speed, triple move allows up to triple speed, and run allows up to quadruple your speed. All three require full round actions (two move actions is a full round action).

Attacks of Opportunity
Attacks of opportunity are often the hardest concept for players to grasp. We have attempted to simplify them as much as possible, but due to the vague sense of location under which forum-based games are played, it is difficult to properly enact attacks of opportunity.

In combat, you or your opponents may sometimes be forced to do something that puts you in a compromising position or requires so much of you concentration or time (or both) that you let your guard down momentarily. In this brief moment, your opponents can take advantage of your lapse in defending yourself and make an attack out of turn, called an Attack of Opportunity. When ever an opponent uses an action that provokes an Attack of Opportunity, if you threaten them, you may make an immediate single attack against them at your highest attack bonus. There are limitations however. As a basic character, you may only make one Attack of Opportunity per round, no matter how many of them are provoked. This doesn’t necessarily need to be the first one provoked either, if you have a very good reason of suspecting that another may come up later in the round, you can save your one per turn and attack when someone else provokes one; be careful however, because if another attack is not provoked, you can’t go back and use the one you passed on, it’s gone for good. This can be changed by taking the Combat Reflexes feat which grants you an extra number of Attacks of Opportunity in a round equal to your Dex modifier, with a minimum of 0 extra attacks per round. In general, your DM will keep track of when you’re doing something that provokes an attack of opportunity and when you get to make an attack of opportunity.


The following articles from Wizards of the Coast may be helpful:
http://www.wizards.com/default.asp?x=dnd/rg/20041026a
http://www.wizards.com/default.asp?x=dnd/rg/20041102a