Modern Warfare 3 has a brand new approach to their weapons progression system in the multiplayer portion of the game. So instead of just leveling your character, you will be leveling you weapons as well. Leveling a weapon unlocks attachments, camos, and weapon proficiencies.
These proficiencies are enhancements to a given weapon that are geared towards a specific playstyle. As you can see in the video below, these proficiencies include things like gun kickback, impact, attachments, focus, breath, and stability, but those are just listed for that specific weapon. Each weapon may have proficiencies that are unique to it, that can be unlocked.
Modern Warfare 3 will be available on November 8th for Xbox 360, PS3, and PC.