What is DNS? What is reverse DNS?

Domain Name System (DNS) is a system that translates URLs (websites, mail servers, etc.) into IP addresses.  The Internet works primarily off of IP version 4 (IPv4) addresses, but rather than having to remember each IP, DNS was created to allow for names to translate into IP addresses.  Reverse DNS is the process in reverse, where you begin with the IP address and are attempting to locate the domain name that references it.