I'm assuming your router does NAT (Network Address Translation), which means that your computer isn't actually on the Internet (in IP Address terms) and can't be accessed directly from there.
Be carful not to spread the wrong message.....
See .....
TeamViewer - Free Remote Access and Remote Desktop Sharing over the Internet
and many other similar "legit" examples..... then consider the "dark side" possibilities.
It's easy to create a backdoor in through NAT......
