You ask what is the different of internal and external IP and why is there 2 IP address.
The reason is that your wifi/operator may give you a internal private IP address (The IP adress space is running out!!) but if you need to communicate with other parties in the internet, you need to use a public identity (most likely it will be a shared one).
with this app, you can see both IP address.
anonymous, IP, network info, internal IP, external IP