The Green Shell, the Red Shell, and the Spiny Shell, the three most common types of shell in the Mario Kart series.
Shells (or Koopa Shells) are the items obtainable in every Mario Kart game, usually coming in red, green, or blue, being used mainly to hit opponents. Each shell has its own abilities to help racers. Green Shells and Red Shells can also come in sets of three.
Green Shell[]
The Green Shell.
The Green Shell is a launchable, which when launched from a player, will start moving. The shell takes 10 bounces on walls before it will break. This can be deadly to players ahead as they must dodge it. It is referred to players as the "worst" shell, mainly because it roams freely instead of targeting a player. The shell is green with black lines.
Triple Green Shells[]
There is also Triple Green Shells, where three regular Green Shells usually rotate the racer's kart.
Red Shell[]
The Red Shell.
The Red Shell (or Homing Shell) is a shell obtainable in all Mario Kart games. The basic idea of the shell is it homes into a player in front of the player who launches it. It is not avoidable unless you have an item with you, or it hits a wall. It can only take 1 hit from a wall before it breaks. It is considered the weakest shell. This shell can be useful when in 2nd, as you can hit the player in 1st and pass him/her. The shell has a light red covering with black lines. It is not recommended you launch a Red Shell backwards, as it loses its homing capabilities and just moves in a straight line.
Triple Red Shells[]
There is also Triple Red Shells, where three regular Red Shells usually rotate the racer's kart.
Spiny Shell[]
The Spiny Shell (or Blue Shell) is a very ruthless and powerful shell that tracks down the player in first place and blows up when it hits them. It has two different forms, normal and flying. In Mario Kart 64, Mario Kart: Super Circuit, Mario Kart 7, Mario Kart 8, Mario Kart 8 Deluxe and Mario Kart Tour it is in normal form. In Mario Kart: Double Dash!!, Mario Kart DS, Mario Kart Wii, and Mario Kart Live: Home Circuit it is in flying form. A Mushroom can be used to dodge it. Spiny Shells cannot be thrown backwards (but in Mario Kart 64, they can be dragged behind the thrower's kart).
In its Normal Form, the shell skirts along the middle of the track and knocks over anyone it hits. In its flying form, it flies through the air, can go over shortcuts and walls, and only hits the player in first (unless other racers are too close to the explosion).
Launching a Spiny Shell in 1st place will result in the shell U-turning after a few seconds and hitting YOU. So don't try that unless you are doing this in teams in VS mode.
The Spiny Shell is the shell that critics hate the most due to them finding out how unfair it is.
Bowser Shell[]
The Bowser Shell.
The Bowser Shell (or Bowser's Shell) is a huge shell that is the Special Item of Bowser and Bowser Jr. in Mario Kart: Double Dash!! and in Mario Kart Tour (in Double Dash!!, Petey Piranha and King Boo can use it, too). This large shell is like a Green Shell, but its size allows it to easily knock over races intensely and remain on the track after striking them. This item doesn't appear in any other mainline Mario Kart games, while it does appear in Mario Kart Arcade GP 2.
Black Shell[]
The Black Shell.
The Black Shell is a black-colored shell that appears in the first three Mario Kart Arcade GP games. This shell behaves like a Green Shell, but once it hits a wall, it explodes!!!
Gold Shell[]
The Gold Shell.
The Gold Shell is one precious shell, being made of gold unlike the other shells. This shiny shell is a Red Shell and a Black Shell combined - that's one killing machine!