A random sequence of "tetrominoes" (sometimes called "tetrads" in other countries) - shapes composed of four square blocks each - fall down the playing background.
The goal of tetris is to manipulate these tetrominoes with the aim of creating a horizontal line of blocks without gaps.
When such a line is created, it disappears, and all the blocks above (if any) fall. As the game progresses, the tetrominoes fall faster, and the game ends when the player "tops out", that is, when the stack of tetrominoes reaches the top of the playing field and no new tetrominoes are able to enter. (The exact definition of a top-out varies from version to version).
The seven one-sided tetrominoes in Tetris are referred to as I, J, L, O, S, T, and Z. All are capable of single and double clears. I, J, and L are able to clear triples. Only the I tetromino has the capacity to clear four lines simultaneously, and this clear is referred to as a "tetris" (This may vary depending on the rotation and compensation rules of each specific Tetris implementation.
For instance, in the Tetris Worlds type rules (see below) used in many recent implementations, certain rare situations allow T, S and Z to 'snap' into tight spots, clearing triples).