This document specifies how DNS resource records are named and structured to facilitate service discovery. Given a type of service that a client is looking for, and a domain in which the client is looking for that service, this allows clients to discover a list of named instances of that desired service, using standard DNS queries. This is referred to as DNS-based Service Discovery, or DNS-SD.

via. http://files.dns-sd.org/draft-cheshire-dnsext-dns-sd.txt