http://www.warriorsofthe.net/

Go there, download the movie, and sit back for some fun. It is a tutorial lesson/guide on how basic TCP/IP works in conjunction with firewalls, routers, and sniffers. Based in a 3d world and using humorus electronic devices, the definition you want is right within that video.

So I'll hold off giving the definition here so you can watch the video, and learn even more about TCP/IP than just routers