Before working with Hydrogen, please familiarize with these file types:
*.h2pattern: XML file describing a single pattern. Patterns are group of beats and are managed in the pattern editor.
*.h2song: XML file describing the whole song (or sequence). Songs are group of patterns with their properties and are manager using the song editor
*.h2playlist: XML file describing a playlist. A Playlist is a (ordered) group of songs.
*.h2drumkit: a compressed and archived folder containing all sound samples composing a drumkit and a description XML file. Drumkits are basically group of sound samples.