An stem file is a discrete or grouped collection of audio sources mixed together. For instance an R & B song might be split into five separate stems: lead vocals, bck vocals, adlibs, guitars, bass, keyboards, horns, strings, percussion, and drums. If each individual track is rendered as a separate audio file, for instance each drum sound, these may also be referred to as parts. Creating remixes of tracks or having tracks mixed by a mix engineer are common instances where you would need to work with or create stems/parts.