I recommend that you see What is an IP? before reading this explanation.

When someone tries to connect to you from outside your network, they may try to connect to your IP address, or your domain if you have a domain pointing to your IP. Once they get to your IP, they have reached your router. Now your router sees that someone is trying to connect on port 80, which is a fancy way of saying someone wants to reach a web server.

The router now must decide what to do; where should it tell the request to go? Well, if your router is configured correctly, it will tell the machine that is trying to connect to go to a certain computer.

The way I have set up this website is so that anyone who tries to connect to http://techxonline.net reaches my domain, which tells them to go to my outside IP address (see What is an IP?). Once they reach my outside address, my router sees “Oh, someone is trying to see a website. Let me send them to the server running the website.” So, it sends the request to the server. If the server computer is configured, which mine is, it sends back the website to the person connecting.

For information on how to setup port forwarding on your router, see this excellent site with step-by-step guides for almost all routers.