Understanding CMB and CMD

CMB and CMD are ways that Paizo worked in to simplify combat maneuvers (disarm, grapple, and so on). CMB stands for Combat Maneuver Bonus, and CMD stands for Combat Maneuver Defense. When executing a combat maneuver, your character will roll his or her CMB against the opponent's CMD. There are no more extended, opposed roles.

CMB

CMB = BAB + STR + Other

From the PRD: When you attempt to perform a combat maneuver, make an attack roll and add your CMB in place of your normal attack bonus. Add any bonuses you currently have on attack rolls due to spells, feats, and other effects. These bonuses must be applicable to the weapon or attack used to perform the maneuver.

This means that weapon enhancement bonuses, and feats such as Weapon Focus apply. It also means that characters using TWF will take a -2 penalty to their roll.

CMD

CMD = 10 + BAB + STR + Size Mod + Other

From the PRD: A creature can also add any circumstance, deflection, dodge, insight, morale, profane, and sacred bonuses to AC to its CMD. Any penalties to a creature's AC also apply to its CMD. A flat-footed creature does not add its Dexterity bonus to its CMD.

...Combat maneuvers are attack rolls, so you must roll for concealment and take any other penalties that would normally apply to an attack roll.

In other words, you can add whatever bonuses you normally apply to your Touch AC. Like Touch AC, if you're flat-footed, you lose your Dexterity bonus. If your character has any current AC penalties, these will also apply. So, CMD is actually:

10 + STR + DEX + BAB + Special Size Modifier + deflection + dodge + insight + morale + profane + sacred - any penalties at all to AC

Overall, the rules require an adjustment, but are simpler in the long run. While the above may appear complex, we're given a single set of numbers apply to every type of maneuver, from disarms, to grapples, and so on. From here on out, it's also a static number unless you change weapons or fighting styles--and then it's a case of remembering you're holding a +2 sword instead of a +1 so the number would then be 1 higher.