(lifted from borderlands wiki)
Class Mods (or COMs, as in COMmand Decks) are a major item in Borderlands, obtained at around level 15. "Class Mod" is short for class modification.
Basically, it's a piece of equipment that will start showing up around level 15 in loot drops or in the medical vendors. You can equip only one at a time and carry others in your backpack.
In the basic game there's something like seven types of COMs per class. Each COM type has a specific primary bonus which will always show up but the amount of the bonus can vary wildly. The COM will almost always have a bonus to a skill up to a +3 bonus. Better COMs will have similar skill bonuses to a second skill or even a third. Like the primary bonus, the skills that a particular type of COM will increase are preset. An important note: these skill bonuses will only increase a skill that you've got at least one skill point in. From my understanding, these bonuses can push a skill's effect over the five point limit that you can put in, however I'm not sure of this. Even so, some skills get no effective bonus effect beyond the five points invested. Also, seldom, there will be one of a possible two secondary bonuses on a COM, but never both secondary bonuses.
As a specific example, there is a COM for Mordecai called Gunslinger. It's primary bonus is an increase in pistol fire rate. The three skills that can potentially get bonuses are Deadly, Loaded and Relentless. The secondary bonus can be either pistol ammo regeneration or an increase in pistol accuracy.