Discovery daemon (discd) is an implementation of the "Discovery Proxy for Multicast DNS-Based Service Discovery" specified as an internet draft of the IETF:
It dynamically maps multicast DNS services into the unicast DNS name space. This allows remote service discovery requests to be answered when the querier is not on the same link local network as the service provider.
Traditionally, zero-conf networking (or Bonjour as Apple Computer calls it) only provides answers to queries on the local network. This works great for home and small offfice (SOHO) networks but frustrates corporate and campus users when a device they want to communicate with is on another IP subnet for network scaling reasons.
By dynamically mapping the ever changing multicast DNS (mDNS) services into the unicast DNS namespace, it's possible to remotely query services through a proxy.
discd is that proxy.