In easy terms, your ISP assigns you one IP addy, as you only have one internet connection. So your routers WAN port will hold this.

Behind the router on your network you could have any ammount of computers on it, all needing internet access. Say you had 50, it would be very unpractical to assign all 50 computers an external IP addess (one from your isp) so they all get an internal IP (192.168.x.x) for use purley and exclusivley on your network, however when they transmit data outside of your network, obviously the traffic has to go through your WAN port on your router.

The WAN port already has the IP addy assigned to it by your ISP so it attached this to the data packet and you now have a external, routable IP addy!

In a nutshell NAT!

To find your external IP log onto your router and look for the WAN IP Address on your status page.

Hope it made sense to you!