Red Shells are items featured in the Mario series, first appearing in Super Mario Bros. for the NES. Koopa Troopas wearing Red Shells were considered cowards as they weren't willing to jump off ledges to defeat Mario. Rather, they would just walk back and fourth on the platform they were standing on. Like Green Shelled Koopas, the shell can be removed and then kicked just by jumping on them in the sidescrollers.
In the Mario Kart series, Red Shells were items that you could find within item boxes. Once tossed, they would home in on the closest enemy. This particular item has been featured in every Mario Kart game to date starting with Super Mario Kart. Interestingly, in Mario Kart: Double Dash‼ for the GameCube, a red shelled Paratroopa was featured as a playable character. His special item was three Red Shells.
In Super Mario Galaxy, Red Shells were the ideal item for underwater usage. During the boss fight against Kingfin, the Red Shells would home in on the enemy where as the Green Shell would just go straight.
Super Smash Bros. series
Red and Green Shells appear in the Super Smash Bros. series, starting with the first game in the beloved franchise staring all of Nintendo's major properties. The Red Shell would, when thrown, follow the enemy's opponents and attack them, whereas the Green Shell would just head forward without tracking onto anyone in particular. While the red shell was present in both the first installment and Super Smash Bros. Melee, it was taken out of Super Smash Bros. Brawl.
Throw a Red Shell, and it will home in on random characters and send them flying straight up. Red Shells moving around the surface are dangerous to all players, but they may also present attack chances for players with good timing and strategy. Red Shells won't leave the platform they're on once they start moving.