Combat Maneuvers...are well, maneuvers. They take a little more finesse to complete than swinging a sword around.
There are two things for Combat Maneuvers - the CMB (Combat Maneuver Bonus) and the CMB (Combat Maneuver Defense).
There are different feats for different kinds of maneuvers, and each of those feats add bonuses to specific types of maneuvers. For example - Improved Disarm gives you a +2 bonus to your CMB for attempting disarms, as well it gives you a +2 to CMD against disarms since you're trained to recognize a disarm.
So mostly, I believe, if you want a character who does a lot of tripping, sundering, disarming, etc. - you'd focus a lot on your CMBs.
Another big thing is that your CMD is different than your AC - CMD is like your AC against Combat Maneuvers. Typically it will be much harder for someone to perform a Combat Maneuver on you unless they're trained (or just if they're a fighter attempting these maneuvers on say, a wizard - higher BAB classes tend to have the advantage). CMB being offensive uses your BAB+STR Mod+Size Mod and whatever feat/bonuses you have and then a roll of the dice added VS. CMD (10+STR Mod+Dex Mod+Size Mod).
I'm not sure if that really answers your question - I don't have my 3.5e books with me to compare as to what the advantage is of splitting the two. But I hope it helps a little.
...and BTW, I've got a group to play in now - so I'm good on the LFG.