Ptr RecordsPtr Records is a setting in DNS that tells you where a specific IP address points to. There are many different protocals that will look up this setting in DNS. Email Spam Filters use this setting to determine if the email is spam or not. You should only have one PTR Record per IP address.
In addition, PTR Records is also know as Reverse DNS or IN ARPA records. Most hosting companies won't give you access to change the PTR records yourself directly in DNS, but if you ask you hosting company to add them for you they normally will.