Dog tags are identification tags worn by army soldiers; they are used for the I.D of dead or wounded soldiers.
As well as helping identify the soldier, dog tags have personal necessary medical information, like the fighters blood type or religious preference's, etc. they are made from a thin piece of aluminium, dog tag silencers are put on them to stop the jangling and giving away your position. Although it's mostly the military that wears them, they have become a staple fashion accessory with lots of pop stars, models and rappers wearing them nowadays. They are cheap to buy and will give your gear that authentic army look.