You can only grapple an opponent up to one size category larger than you and only one opponent at a time.
A grappling attack is treated as an unarmed attack except that it deals no damage. If the grappling attack hits, you and your opponent immediately make opposed grapple checks. A grapple check is 1d20 + base attack bonus + strength or dexterity (whichever is higher) + size modifier (small -5, medium -, large +5). If your check result equals or exceeds the targets check result, the target is grappled.
You can choose to pin or trip a grappled target.
A pinned creature can’t move or take any actions while pinned, and it looses its dexterity bonus (if any) to Reflex Defense.
You may maintain a pin by making another opposed grapple check on your next turn.
If you are armed with a light weapon you may deal damage with that weapon if you pin a creature, no attack roll is necessary. You can continue to deal damage on your turn as long as you can maintain the pin.
Instead of pinning you may chose to trip a grappled target. A tripped creature falls prone in its square and is no longer considered grappled.
feat: crush
prerequisite: base attack bonus +1
You can deal unarmed damage to a pinned creature as if you were armed with a light weapon.
feat: throw
If you successfully trip a creature it falls prone in any unoccupied square you desire up to 1 square beyond your reach and you can deal unarmed damage. The creature is no longer considered grappled.