Forward resolution turns DNS names into IP addresses. Reverse resolution turns IP addresses back into names. The folks who wrote the DNS specifications decided it would be best to use the same software logic for both forward and reverse resolution. To make this work, they decided it would be best to store all the names under nodes labled by IP addresses, and to place the IP address nodes under a special top level domain: arpa.
Under the arpa domain is the in-addr sub-domain, which is a shortened form of 'inverse-address'. Today, it is very rare for anything else other than IP address inverse resolution information to be stored under the arpa top level domain.
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