The master server is the central part of Won2 or most any game server network. Constantly replying to requests coming from the ever shifting community, the master is responsible for ensuring upstarts in the server business get their fair share of info queries.Lookie Here.
Masters are by no means a mysterious concept. You can read up on master servers in this nicely written article, which discusses networking, requirements, and potential pitfalls. A must for the aspiring master writer.
Other resources:
The Sourceforge master, aka bullet's master
The No-Steam master (no relation to Steamless, but that's another story.)
Tomi, and his master server aggregator