on college helmets it designates BMOC for the current week. he gets his run of the sororities (or in the case of john david booty...fraternities).

the buckeyes, and other teams put stickers on the helmets to validate good plays in games, or in practice, basically its like a gold star that you get on your test when you get an A, in 2nd grade.

how many key plays they made and how good they are doing at practice

It's kinda of like getting a game ball. At the end of the game, the stickers are awarded to players who made great plays during the game.

They are for plenty of different things depending on your position. Could be mvp in a game, a huge play or tackle, touchdowns or interceptions. Just depends on the coaching staff. Not all teams use them though as you have probably noticed. Teams like Michigan don't use them because they prefer to win as a unit rather than continuosly reward the same people for individual performances. A whole lot of stickers on one player's helmet isn't so great for morale of other players who may ride the pines most games.

"Atta boy" stickers.