Playdar is designed to solve one problem: given the name of a track, find me a way to listen to it right now.
It will search your local disk (iTunes library, MP3 folder etc.), it will search your home or office network (kinda like searchable iTunes shared libraries) and it will search other sources. You can write plugins for Playdar to enable it to search additional sources. It's fast. If a matching song is within reach, it can start playing in less than a second.
This is called Content Resolution. Playdar is a "Content Resolver" - it will take metadata (artist, album, track names) and resolve them to a location of a matching music file. This might be on your disk, or over the network - regardless, sources are always presented as URLs that point to your local machine. Playdar runs a lightweight webserver on your machine - this is how you talk to it, and it's how other desktop apps and web apps talk to it.