A UN*X program for dealing with multiple archive formats in a single program. pax supports cpio, ustar (IEEE 1003.1(1990)) and sometimes xtar (which is like ustar, only might support a few vendor-specific file properties).
What it really does is offer a nice uniform front end to cpio and tar (imagine having the same options to both!). For some reason, it's never found favour with users. Speaking for myself, the pain of learning to use tar and cpio effectively was so great that I don't want to learn another set of options, even if they are rational!