Standard Tetris has seven tetromino shapes. Each tetromino is formed from four connected square blocks, and the seven free tetrominoes are commonly represented by the letters I, O, T, S, Z, J and L.
Alexey Pajitnov originally drew inspiration from pentomino puzzles, which use five squares and have twelve free shapes. He decided that twelve shapes would make the computer game unnecessarily complicated, so he reduced the pieces to tetrominoes. The four-square pieces made the rules easier to learn while preserving plenty of strategic variety.
The seven shapes are not the same as seven rotations. Several pieces have multiple orientations: the I piece has two distinct rotations, while the T, J, L, S and Z pieces have four in ordinary play. The O piece looks unchanged when rotated. These orientations are positions of the same shape, not additional tetrominoes.
Modern Tetris versions often use a random “bag” system that guarantees one complete set of seven pieces before any piece repeats. That distribution rule is newer than the original game, but it is why players commonly encounter all seven shapes in balanced groups.