From wiki
The object of Tetris is to last as long as possible before the screen fills up with tetrominoes. To do this, you must assemble the tetrominoes to form one or more rows of blocks that span the entire playing field, called a line clear. When you do so, the row will disappear, causing the ones above it to settle.
[edit] Types of line clears
[edit] Single
A single is the act of clearing one line at a time:
[edit] Double
A double is when two lines are cleared at once:
[edit] Triple
A triple is three lines cleared simultaneously. Most games allow only I, L, and J tetrominoes to complete a triple, but newer games with SRS allow S, Z, and T tetrominoes to twist into seemingly impossible positions.
[edit] Tetris
A tetris is four lines cleared simultaneously. In most games, this can only be done with an "I" tetromino.
