Defender was one of the very first games for the Atari 2600.  You pilot the spaceship defender.  Your mission is to defend the humans from aliens who have come to take over the world.  You can move left, right, up, down and at an angle through the scrolling screens of each level.  You must be traveling in a circle though, because if you keep traveling in one direction, you end up back where you started!  There are a variety of enemy spacecrafts for you to shoot.  One type of spacecraft picks up the humans.  When this happens, you have to shoot the alien ship and either let the human fall back down to earth (if he is low enough) or fly by and grab him.  Don't worry, laser blasts from you and from the aliens won't hurt the humans.  If the alien carries the human all the way to the top of the screen, the human mutates into an alien which you must kill.