: A digital collection of piano scores, often compiled by an individual or a specific online community.

: Raw .wav or .sf2 (SoundFont) files of a specific piano for use in a sampler.

: Extra patches or "bonus" sounds for popular piano plugins like Keyscape, Addictive Keys, or Kontakt.

If you can tell me or what software it is meant for, I can likely give you a much more specific list of what is inside.