COG tags are the GoW equivalent to dogtags and are found in various hidden locations throughout the game. Tags are cumulative, so once a tag is acquired it will not be present during subsequent play throughs on any difficulty. However, if you are playing in co-op and your partner has not picked up a tag, you will be able to see it but not pick it up. If neither of you have picked up the tag, only one of you may pick it up and have it count toward your total.
There are 30 total COG tags in the game (33 for the PC version). See the COG Tags page for the locations of every tag.