An "External IP" is the IP Address that your ISP (Internet Service Provider) asigns you so that you can go online. Most ISP companies change your IP address from time to time, if this is your case, you are assigned a "Dynamic IP".
If you always have the same IP address, you then have a "Static IP".

In most cases, your computer does not have this External IP, the External IP is assigned to your router, cable modem, DSL modem or firewall.
Your router is the one that assigns your computer an "Internal IP". It is very dificult for a website to see the internal IP address (not impossible).
If you would like to find out the IP of your computer, Click here.