If DNS is used to translate human-readable Internet addressses (www.fluidhosting.com) into numerical IP addresses (188.8.131.52) basically, rDNS or Reverse DNS does the exact opposite. It resolves a known IP address into an associated Internet address or hostname.
One of the important usage of rDNS is in anti-spamming. E-mail anti-spam method is to verify the domain names in the rDNS to make out if they are likely from dsl or dialup users. Because most of the e-mail that comes from these computers are spam, most mail servers refuse e-mail with missing or "generic" rDNS names.
Another would be for network troubleshooting tool, such as ping, traceroute and the "Received:" trace header field for the Simple Mail Transfer Protocol (SMTP).