A Green Shell is a green Koopa shell that can be used as an item in different Mario games. They are either used as projectiles or ridden as a means of transportation. There also exist Red and Blue Shells.
Super Smash Bros. series
Similar to its role in the original Mario games, the Green Shell is an item that can be thrown. It continues until it hits a wall when thrown or jumped on, harming characters it hits, and falls off edges it meets. It will often deal multiple hits if it rams into an opponent after he/she falls from the first hit.
Mario Kart series
The Green Shell in the Mario Kart series is also thrown, either backwards or forwards, and bounces off walls. It only breaks when it hits a racer, at which they are slowed down. In this series, the Green Shell can also be obtained in a set of three, an item known as the "Triple Green Shell", which, when used, constantly circle the racer and can be launched one by one. The Green Shell has appeared in every single Mario Kart game.
2D Mario platformers
In games such as the Super Mario Bros. series and Super Mario World, Green Shells are found on Green Koopas; these, when jumped on, withdraw into their shell. When jumped on, the shell moves along the ground, defeating most enemies and bouncing off walls, and activating any Question Blocks it slides into from the side. It disappears only when it moves a certain distance off-screen.
Koopa with green shells will fall of an ledge when they get to the end of one, this is different from Red Shelled Koopas, whom will turn around and begin walking the opposite direction when reaching a ledge.
Super Mario 64
In Super Mario 64 (and Super Mario 64 DS), Green Shells can be found in Exclamation Blocks and ridden. They allow Mario and other playable characters to ride over the surface of lava, water and sinking sand as well as normal surfaces, and can be ridden up relatively steep slopes. While using it, the player can defeat most enemies by coming into contact with them. The shell breaks if it hits a wall, and can be broken by the player while on the ground at the press of a button.
Mario & Luigi Series
In Mario & Luigi: Partners in Time, the Green Shell is a Bros. Item that is used as a weapon against enemies. At least two bros. are required to use the item. When the item is chosen, one of the brothers will kick the Shell towards the foe(with ,, or that represent that bro.). It will then bounce off the enemy, cause damage, and come to the other player. After each kick, the shell starts moving faster along the ground. The brothers can then repeat this until the player misses a kick, the enemy is defeated, or sixteen successful hits are dealt; in any one of these cases, the Green Shell disappears. When the adult and baby Mario brothers fight together, the babies can sit over the shell and spin upon hitting the enemy, causing extra damage to the foe(with or with baby Mario or baby Luigi respectively).
Striking two to four, five to seven, eight to nine and ten times with the Green Shell will yield an "OK!", "Good!", "Great!" and "Excellent!" rating respectively.
Super Mario Galaxy
In Super Mario Galaxy, Koopas are found wearing Green Shells; harming them allows Mario or Luigi to pick up and throw the shell. It harms enemies and breaks upon coming into contact with anything solid.