Bind is the ISC name server, the most commonly dns server in UNIX and Linux. There are another name server like Microsoft name server (IN Windows Server), dnsmasq, djbdns and more.
I will talk about only ISC-Bind named server.
Did u use IP address on the internet? no, only address name. But internet infrastructure can only serve and tell you how to access a server with IP addresses, so you must have a directory where you can have name with IP addresses who can tell you what is this IP and where is this name.
In short terms, a DNS is like /etc/hosts but in client-server mode.
In the example file, we use a small and minimal config.
we have 2 parts
This is a key/value database with name as key and IP as value.
Same as above, but IP as key and name as value. Usefull when you have a IP and want to know what is it.