PRC started out as generic PALMOS extension for a variety of data files.
In the ebook arena, PRC is a file extesion for Mobipocket files and can be created by Mobipocket Creator (free at mobipocket.com) or other tools like MobiPerl or Calibre. The format can also use the extension .mobi and (if DRM-free) .azw for use with Kindle ebook readers and apps.
For more detail:
http://www.mobipocket.com/dev/articl...mobiformat.htm