There are two sources of points :
Dropping down a piece gives a small amount of points which is equal to the drop height. It is not much each time but better than one can think perticularly in small levels (when the pieces fall slowly).
The biggest part of the score comes from destroying full lines. It is very important to note that destroying multiple lines at the same time gives you much more points. A four-lines destruction is called a tetris : it gives a maximum amount of points.
The points gained are 40 for a one-liner, 100 for two lines, 300 for three lines and 1200 for a tetris. These numbers must be multiplied by the current level. Destroying four lines in level 10 gives 12000 points !!