There are 5 ribbons in Killzone 3. They can only be done in the multiplayer.
Ribbons
Faster Aim - Aim down sights faster. Requirements- 10 ADS kills
Faster Reload - reloads faster with each weapon. Requirements- 10 hipfire kills
Bullet Damage - Increase bullet damage. Requirements- 5 assists
Increase Accuracy- Make your crosshairs smaller for each gun but explosives and shotguns. Requirements- 3 melee kills, brutal or not.
Double XP - Get more XP for everything you do for a limited amount of time. Requirement- Complete an objective