Problem scenario
You have heard of A records and PTR records. But you do not know the difference. What is the difference?
Solution
The short answer is that they are opposites of each other in the context of DNS.
An A record maps a domain name to an IP address (pg 858 of A Practical Guide to Fedora and Red Hat Enterprise Linux).
A PTR record maps a host name to an IP address (pg 859 of A Practical Guide to Fedora and Red Hat Enterprise Linux).
They are complementary. Depending on what you need to find and what you have (e.g., a host name vs. an IP address), the two can help you configure your network.