Each rpp contains an archive and a bunch of other information. The archive is simply the collection of files the package contains. The other information includes: name, version, type (source or binary), size (and size to build for source packages), dependencies, date built and builder, and the description.
The file name convention for rpps is name- version- {bin,src}.rpp. This is only a useful convention, the file names can be whatever you like, but it does go a long way towards making the file names unique.