Decals are stickers, which mostly include logos from car tuning companies. In Need for Speed, decals could only be put on certain parts of a car (doors, fenders and bonnet), but since the release of Need for Speed: Carbon, they can be moved anywhere.
BILSTEIN Decal on a Subaru Impreza WRX STI in Need for Speed: Carbon.
Decals can be toggled white or black.
In Need for Speed: Underground 2, decals raise the player's current car's star rating by .01.