NAT on a router... what is it?

i was told that "NAT" acts as some sort of fire wall, i dont know what NAT is
and when i disabled it my computer couldnt connect to the internet..
im still trying to figure out why i still have firewalls running when i have turned all of them off, its preventing me from connecting to certain servers

 

May be this can Help, http://www.ezlan.net/routers1.html

And this, Basic Protection for Broadband Internet Installation.

 

NAT - network address translation
translates your private address into a public address and vice versa