COG tags are the future's equivalent of dogtags. They are hidden throughout the game and can be picked up in various chapters of every act. You will notice them by their illuminated blue glow. If you have picked up a COG tag in a certain area, and are playing through the game again, you will not be able to pick that COG tag up again. The number of COGs you have picked up can be seen in the pause menu.
For a list of all COG tags, click here.
If you are playing in Co-op, you will be able to see a COG tag you have already picked up provided your partner has not done so. However, you will not be able to pick it up.
The cog tags can be picked up in Co-op. However, only one player can pick up any given tag.