The main class is the QTunes Server, which parses the configuration file specified on the command line. This is an easily understood format, and mostly all it does is specify which Services to load and what their properties are. The various subclasses of Service interface (eg org.qtunes.scanner.Scanner, org.qtunes.daap.DaapServer, org.qtunes.webplayer.WebPlayer) actually do the work.
Each Service has its own package, and the "spi" packages provide implementations, or sometimes multiple alternate implementations, of an abstract Service API