tt is an implementation of the well-known game tetris.  Quadominoes
(groups of four squares joined orthogonally together) fall slowly down
the screen, accumulating at the bottom, and when the pile reaches the
top of the screen the game is over.  The pieces may be moved to the left
or right, and rotated as they fall, with the aim of making them
tessellate with the pieces already at the bottom of the game area.  The
height of the stack of pieces can be reduced by filling a complete row
of ten squares, at which point that row will disappear, and those above
will fall down into its place.  It is possible (and desirable) to destroy
multiple rows at once.

WWW: http://www.miketaylor.org.uk/tech/tt/