PTR, or Pointer records are usually described as the opposite of an A record. While A records point the domain to an IP address, a PTR record points an IP to a domain. This is commonly used as spam verification with certain email programs to confirm a mailserver is really authorized to use the domain the email is coming from. PTR records usually have to be defined by the owner of the IP address for your server, usually your server hosts. Many hosting companies will set this up for you when you set up a server.