Each release has a version number that's strictly monotonic increasing. In addition to that, all releases starting from v0.9.5 has a unique code name — a single word (e.g. "Butterfly") or a short phrase (e.g. "Watcher of the skies"). People are wondering what's up with those code names.
Those code names are actually names of songs, to be more precise: songs that I currently like when I'm preparing the release. They're from a diverse range artists and bands, but I'm mostly into Progressive Rock, vocal pop/Jazz music (both choirs and ensembles), Funk, Jazz and Singer/Songwriter.
There's also a list of all the releases including their code names and the artist performing those songs available online as an XML document. You can find the artist and the songs as the attributes codename-artist and codename to the release element.