(I don't think this article is necessary if you're playing D&D 3.5, as IIRC they did a similar breakdown for the grapple rules in the 3.5 Player's Handbook.)

Grappling in D&D 3.0, Step By Step

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After a talk with a friend this weekend, I realized that grappling is still sorta confusing as written in the Player's Handbook. This is a step-by-step procedure for handling grappling, hopefully easier to understand. If there's anything wrong in this, let me know; I studied the section before I wrote this, but parts of it are still confusing, even for me. :)

Items in red are normally reserved for monsters with certain special abilities. Items in green are special cases or exceptions 

1. Attacker attempts to start a grapple.
2. Defender gets to make an AOO against the attacker. If the AOO deals damage, the grapple attempt automatically fails; otherwise, continue to the next step.
3. Attacker makes a melee touch attack against the defender. If the touch attack fails, the grapple attempt fails; otherwise, continue to the next step.
    Monsters with improved grab may automatically skip steps 1-3 whenever they make a successful melee attack.
4. Attacker and defender make grapple checks (d20 + BAB + Str mod + special size mod). If the defender wins, the grapple attempt is ended. Otherwise, continue to the next step.
5. The attacker has a hold on the defender. The attacker deals unarmed strike damage to the defender. To maintain the grapple, the attacker must move into the defender's space, provoking attacks of opportunity normally for movement except from the defender.
    Monsters with improved grab don't deal unarmed strike damage on the round they makes a successful grapple because they already did damage with their normal melee attack.
    Monsters with improved grab and constrict do deal their constrict damage on the attack that resulted in the hold. Pretty harsh.
    Monsters with improved grab pull the target into their space instead of moving into the target's space. The mosters are not considered grappled, so they don't lose their Dex bonus, still threaten an area, and can still make other attacks in the round normally.

6. The two characters are now grappling.
    Attacker and defender are denied their Dex bonus to AC while grappling. Ranged attacks against either have a 50% chance of striking the other grappler. These penalties only apply to attacks made by people not involved in the grapple (in other words, attacker and defender attack each other normally).
    Attacker and defender have no threatened area while grappling.

Steps 1-5 all take place within the course of a single attack by the attacker.

If you are grappling, you can do the following things with a successful opposed grapple check. Each one counts as an attack (so if you have only one attack per round you have to do these on successive rounds, but if you have multiple attacks per round from having a high BAB you could do these as followups to the attack that started the grapple in the first place). Monks may use their favorable attack rate for grapple attacks.

Damage your opponent as if with an unarmed strike.
    It doesn't matter if you're the one who started the grapple or not; once you're grappling, on your turn you can choose to harm your attacker or try to escape (you're not forced to only defend yourself just because you were the defender in the initial grapple attempt). It's quite possible for the defender to successfully damage or pin the original attacker during the defender's attack sequence.
    You can opt to do normal damage instead of subdual if you take a -4 on your grapple check.
    You can perform bull rush, disarm, and trip attempts against your grappling opponent.
    Monks can deal normal or subdual damage without a penalty to their grapple check.
    Monsters with the improved grab and constrict ability deal the damage listed for the attack that caused the hold (they don't do unarmed strike damage).

Pin an opponent, holding them immoble until your turn next round.
    As above, this is also an option for the defender in the original grapple. In other words, you can "turn the tables" on your opponent.
    If you have attacks left after doing this, normally you'd use them to damage your opponent as above.
    You can't damage or pin a second creature while maintaining a pin on the defender.
    When pinning someone, melee attacks against the defender are at +4, ranged attacks are at -4. This modifier only applies to attacks made by people not involved in the grapple (in other words, attacker and defender attack each other normally).
    When you have someone pinned, you can move them if you are strong enough to carry or drag them (so you could carry or drag them to another location within range of your movement for that round).

Break a pin, either on yourself or another pinned creature.
    If you break the pin, you (or the creature you helped) are still grappled. If you're the person who started the grapple, you don't want to do this (this is an action the defender takes to try to end the situation).

Escape from the grapple.
    If you do this, the attacker and defender are no longer grappling, and if either wants to start grappling again they have to go through steps 1-6 again. If you're the person who started the grapple, you don't want to do this (this is an action the defender takes to try to end the situation).
    If you are pinned and you have multiple attacks in a round from having a high BAB, you can use one attack to break the pin and another to escape the grapple, completely freeing yourself.

Attack with a light weapon.
    Because you're using one arm to maintain the grapple (or someone is holding one of your arms to maintain the grapple), you can't use two weapons while grappling.
    You can't do this while pinned or pinning.

Cast a spell.
    The spell has to be 1 action or faster, have no somatic components, and you have to have in hand any material components needed for the spell. You have to succeed at a Concentration check (DC 20 + spell level) or lose the spell.

Wriggle Free.
    You can make an Escape Artist check as a standard action to escape a grapple or get out of a pin. Your check is opposed by the opponent's grapple check. If successful, you can also move.
    {In my opinion, you ought to be able to use Escape Artist instead of your opposed grapple check whenever you're trying to avoid being grappled or pinned, not just when they already have you, and you ought to be able to do it as an attack (not as a standard action). Otherwise the skill just sucks (unless you max out your Escape artist skill, your opposed grapple check is going to be about the same as your Escape Artist check, and since you can do an opposed grapple check once per attack instead of once per round you're better off making multiple attempts per round instead of just one).}

End The Grapple
    The person who initiated the grapple can end the grapple as a free action. If the defender also wants to end the grapple, no roll is needed. If the defender wants to continue the grapple, the attacker has to try the "escape from the grapple" action as described above.