Syntax (example)

123.123.123.123

About

A reverse DNS lookup is when you ask the authoritive DNS for a given IP-address about the PTR record for the IP.

So just to be clear: It is not a different angle on resolving a hostname, it is really something completely different. Even though the results of the two lookups will often have some relation to each other, they do not need to at all.

Simplified: When you resolve a hostname into an IP you ask the DNS for the domain(host)name for an A(address)-record which is an IP-address if it exist. When you are doing a reverse lookup you are asking a DNS server for a given IP (often a range) for a hostname (domain name) in the for of a PTR record for the given zone.