Dog Tags

Dog tags are two flat metal tokens attached to a small chain that are carried by military personnel. Each token has the carrier's personal information such as name, date of birth, religion, and serial number. They are normally used to identify soldiers found wounded or dead, especially if the corpse is rendered unrecognizable (face may have been mutilated or damaged beyond recognition).

In-Game
Dog tags can be seen in one instance in Call of Duty 4: Modern Warfare, carried by the dead soldier Paulsen. They are retrieved by Nikolai.

If you die and don't respawn before the end of a match in Call of Duty 4: Modern Warfare you have a pair of dog tags next to your name.

In Call of Duty: World at War, in the level 'Breaking Point' depending on the players actions Sgt Roebuck will give Polonskys dog tags to Miller. If the player saves Polonsky instead then Polonsky gives Roebucks dog tags to Miller. Roebuck/Polonsky was killed minutes before by suicide grenadiers