Wikipedia: Tetromino

A tetromino, also spelled tetramino or tetrimino, is a geometric shape composed of four squares, connected orthogonally. This is a particular type of polyomino?, like dominoes and pentominoes are. Sometimes the term is generalized to apply to configurations of four orthogonally connected cubes.

A popular use of tetrominoes is in the video game Tetris.

Counting rotations in two dimensions as equivalent, there are seven possible shapes:

I (also called "stick", "straight")
```
 _ _ _ _
|_|_|_|_|
```
Square (also called "O", "package", "block")
```
 _ _
|_|_|
|_|_|
```
T
```
 _ _ _
|_|_|_|
  |_|
```
L
```
 _
|_|
|_|_
|_|_|
```
J (also called "inverted L", "Gamma". This piece is a reflection of L but cannot be rotated into L in two dimensions; this is an example of chirality. However, in three dimensions, this piece is identical to L.)
```
   _
  |_|
 _|_|
|_|_|
```
S
```
   _ _
 _|_|_|
|_|_|
```
Z (Reflection to S. See "J", above.)
```
 _ _
|_|_|_
  |_|_|
```

When added the third dimension, there are three more patterns:

left screw (chiral in 3D)
```
 _ _
|_|_|
  |+| unit cube on top of +
```
right screw (chiral in 3D)
```
 _ _
|_|_|
|+|   unit cube on top of +
```
branch (nonchiral in 3D)
```
 _ _
|_|+| unit cube on top of +
  |_|
```

Some people refer to the pieces by the colour in which they are drawn in a particular implementation of the Tetris game, but those colours vary from implementation to implementation so this is not very sensible. For example, in many older versions of Tetris, the red piece is I.

Colors of pieces in various implementations (under construction)
Piece	Vadim Gerasimov's original Tetris	[Microsoft Tetris]?	[The New Tetris]?
I	red	red	cyan
Square	blue	cyan	white
T	brown	gray	yellow
L	magenta	yellow	magenta
J	white	magenta	blue
S	green	blue	green
Z	cyan	green	red

Note that the box art of The New Tetris reverses the colors of the L and J pieces and the S and Z pieces.