Galaxian is one of my favorite Atari 2600 games!  It is similar to the game Space Invaders with a few differences.  In Galaxian, like in Space Invaders, you control a spaceship positioned at the bottom of the screen pointing up.  You can move only right or left and can only shoot up.  Your goal is to shoot all of the alien bug ships positioned in 6 rows at the top of the screen.  You advance to the next level by destroying all of the alien bug ships.  Unlike Space Invaders, the alien bug ships can leave their position in the rows to attack you.  There are two ways to get killed in Galaxian: 1) if you are hit by an alien ship or 2) if you are shot by one of the aliens.  This game is very easy to find cheap, so if you don't have it, pick up a copy!