Shells (or Koopa Shells) are the items obtainable in every Mario Kart, coming in red, green or blue. Each shell has its own abilities to help players. Green Shells and Red Shells can come in sets of three.
The Green Shell is a launchable, which when launched from a player, will start moving. The shell takes 5 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.
The Red shell (or Homing Shell) is a shell obtanible in all Mario Kart games. The basic idea of the shell is it homes into a player infront of the player who launches it. It is not dodgeble 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.
The Blue Shell (or Spiny Shell) is a very 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, and Mario Kart 8, it is in normal form. In Mario Kart: Double Dash!!, Mario Kart DS, and Mario Kart Wii, it is in flying form. In Mario Kart Wii you can use a Mushroom to dodge it. Blue 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 Blue Shell in 1st place will result in the shell U-turning after a few seconds and hitting YOU. So don't try that.
The Blue Spiny Shell is the shell that critics hate the most due to them finding out how unfair it is.