Firewalls - how do they work?

As I am in incredibly smart person, I found myself wondering about firewalls. And I said to myself, the only thing they really seem to do is block all connections to ports (maybe only the first 1024 anyway). So I wondered, could I make a program that binds to all those ports in the same way, and only 'opens' (doesnt bind to) the ports I specify to be open? If I did that, would it be considered a firewall? I'd like any suggestions and comments on this matter, I'm about to download a free one now and see if thats all it does, and if it is I'll make my own program to do it :)

Its more complicated than that, but I guess you already knew that really.

http://www.howstuffworks.com/firewall.htm
http://www.google.com/search?hl=en&l...=Google+Search

Yeah, I figured there was more to them, such as monitoring internet applications, filtering for content etc, but I was wondering about just the basic concepts. Thanks for the links Hammer, I'll read them now :)