First off, 192.168.x.x is generally internal IP. In short, there are two types of IP addresses in the network.
First is the "external IP". This is pretty much what anyone on your network appears as through the internet. By simply connecting online you are showing off your external IP (and no, you cannot be hacked through an external IP). For example, the Blockland Forums logs every post you make on here with the external IP that the post was written from. If you have two computers in your house, post a message on one and then post a separate message on the other, you'd get the same external IP. If you had a forum account on here, and your brother/sister/etc. had a forum account here, and they got themselves IP banned, you too would be IP banned (this has happened in a few cases).
The second is the "internal IP". It is how the network distinguishes one device connected to the router from the other. Every device that uses the internet through the router has their own internal IP address, your Xbox/Playstation/Wii, computers, even wireless devices like Zune HD's and iPod Touches.
When you try to host a minecraft server without port forwarding, nobody can connect because your router is pretty much going, "What the forget? Who the forget do I send these users to? Which computer? To the TV? Xbox? What the forget!"
With port forwarding, the router pretty much is set up to send those users to your computer where you are hosting a server. So then, your router is now going, "Oh, because the smart clever user port forwarded, I know that I need to send these user connections at this port to this specific computer in the network."
"But how do I port forward?"http://portforward.com/
