That's a question I've been asking myself, and I came to these conclusions:
Pro: A pro is a player who refuses to use anything that gives him any sort of edge. Rather than using a good weapon and taking quick potshots at enemy players, the pro prefers to equip a near useless gun'n'perk setup, then go charging head on into gunfire in a mad suicide attack. He does this as he feels this is the proper way of playing.
Sample quote: "Fuck the fuckin ump iz for pussies and fagz with no skillz, nd only fuckin noobs hide in a fuckin corner with oma nd stopping power. I run around nd only fuckin use my fuckin m9 cuz i am fuckin hardcore."
Noob: A noob is a player who prefers to take a more sensible approach. While the "pros" charge off to get ripped to shreds, the noob finds a good spot, usually at high ground, and sets up claymores, Tactical Insertions, etc. He then takes out his weapon, usually the Intervention or another long-range, powerful weapon, and starts picking off the pros, who are still charging around crazily in the open. As the noob has OMA as a perk, he doesn't need to worry about ammo shortages. The noob typically remains alive for longest during a match.
Sample quote: "It's not my fault you're too stupid to realise charging into the open is gonna get you killed. Damn 10-year-olds..."
So there's my definition of "pro" and "noob". I hope this was helpful to anyone else confused by the terms. ;-)