Space Invaders is a classic!  You control a cannon at the bottom of the screen.  You can move right or left and shoot strait up.  Above you are 6 rows of 6 aliens.  They move as a swarm to the left.  When they reach the edge of the screen, they drop down a bit then move to the right.  They also shoot at you.  You have to shoot all of the aliens before they shoot you or before they land at the bottom of the screen.   

In addition to the basic game play, there were 112 game variations on this cartridge.  Some variations provided you with shields to hide under. In some variations, the shields move.  Some variations have aliens which shoot zigzagging laser bombs or faster than normal laser bombs.  Some even have invisible aliens.  Each variation has both a 1 player and a 2 player option.

Points are awarded as follows:

Row 1 - 05 points
Row 2 - 10 points
Row 3 - 15 points
Row 4 - 20 points
Row 5 - 25 points
Row 6 - 30 points

Although is sounds like a pretty simple game, it is the blueprint for dozens of games that were created in the wake of it's success.  I really enjoy playing this one!  Very easy to find cheap!

Screen Shot from Instruction Manual