A popular use of tetrominoes is in the video game Tetris.
Counting rotations in 2d as equivalent, there are seven possible shapes:
_ _ _ _ |_|_|_|_|
_ _ |_|_| |_|_|
_ _ _ |_|_|_| |_|
_ |_| |_|_ |_|_|
_ |_| _|_| |_|_|
_ _ _|_|_| |_|_|
_ _ |_|_|_ |_|_|
When added the third dimension, there are three more patterns:
_ _ |_|_| |X| unit cube on top of X
_ _ |_|_| |X| unit cube underneath X
_ _ |_|X| unit cube on top of X |_|
Some people refer to the pieces by the colour in which they are drawn in the game Tetris, but those colours vary so this not very sensible. Nonetheless, the red piece is I.