There are many ways you can use and manipulate DNS, most of which are available to you within your web host control panel. When you create a domain normally you will be given an IP address for it. This IP address represents the website or the server the website is located on, depending on if you are hosting your domain on a shared hosting platform or a dedicated server. These DNS records are kept locally on the name servers of the host.
Understanding that DNS defines where your website is located is essential to knowing how to control multiple domains and pointing them to the same website even. For instance, you buy the domain with a .com, .net, and .org extension and you wish to point them all to the .com address. You will do this with your DNS records. DNS also controls other things besides websites since it identifies devices by IP address so even e-mail servers which leads to why you need reverse DNS.
Reverse DNS is generally used for email services, it does the exact same thing as DNS except in reverse. Basically, when a DNS request gets to a name server it will do a reverse DNS look up which takes the IP address and then finds the domain name.
The reasons for using reverse DNS is because it is a security feature as well as protecting against spammers.
It is a simple test to make sure any communicating DNS server has set up their servers correctly.
It helps prevent spam because most mail servers will do a reverse DNS look up and then if it fails they place it on a greylist.
Greylisting means that if it is a legitimate email they will simply need to resend the email and it should go through to the correct email. Using DNS and rDNS is essential to making the internet work and is constantly happening all of the time throughout the world!