From the Super Mario Wiki
Spiny Shell is the Spiny variety of Koopa Shells which are worn by Spinies. The shell, however, cannot be picked up or thrown, as Mario and Luigi are unable to jump on it, and throwing another shell at the Spiny defeats it. However, in the GBA version of Super Mario Bros. 3 and also in New Super Mario Bros. Wii, it is possible to carry an upside-down Spiny Shell.
They are not to be confused with Blue Shells, as the sticker (a Blue Spiny Shell with wings) in Super Smash Bros. Brawl is called a Spiny Shell, and the Mario Kart series item is also called a Spiny Shell.
The Spiny Shell icon in Super Mario Strikers
In Super Mario Strikers and Mario Strikers Charged, Red Spiny Shells act as items. When fired, they will shoot towards the other players, acting very similar to the Green Shells. However, instead of just hitting one player, then disintegrating, the Spiny Shells will keep going. This can be compared with the Giant Green Shells. They continue to bounce around the stage until, at long last, they wear out, and get destroyed on their own. The more effective counterpart of the Spiny Shell, is the Giant Spiny Shell. Spiny Shells usually come in groups of three.