AAAA is used with Internet Protocol Version 6 (IPv6). IPv6 is at some point will take the place of IPv4 the first Internet Protocol. The reason for this, is the depletion of available Internet addresses. IPv6 uses a 128 bit address which is much larger then IPv4 which is a 32 bit address. Hostnames are mapped to IPv6 addresses by AAAA resource records, which are also called quad A records.
See also:
A Record
Hostname
Resource Record