Video games are interactive movies that put you in the heart of the story. Your push of the buttons and moving of joysticks determine how you guide the main character through the story. Whether that story is running around defeating monsters, and completing quests, helping your team get to the finals, flying through space in a one-man war to stop an evil empire, collecting and racing the hottest cars, being the star in your own rock band, or simply organizing blocks into rows.
The motivations for playing are many. Some like to explore the vast world laid before them, others the lust of being a near-unstoppable engine of destruction, the likes of which the real world might not tolerate. Some games even allow players to compete against each other in arenas where life, death and instant rebirth (also called respawning) fluctuate frequently and points are counted based on the dominant players' skills.
I would recommend simple games to start with. Pac-Man and Super Mario Bros are easy to play and learn for all ages. The next easiest would be any racing game. Newer racing games would most likely have a tutorial to help starting players along. Another simple game line to start with would be any from the Guitar Hero or Rockband series. From there, the sky is the limit.
Not bad for a start eh?