A par file is (it seems) a parity archive. When you're downloading porn off of usenet there is the constant problem of missing parts, and how can you get your jolly boat a'sailin' when you don't have all the pieces of the rar file? In the bad old days you'd beg, plead, and bargain for fills, but now you just need to beg, plead, or bargain for the par files, often posted with the pr0n to begin with. For any missing or corrupt rar file, you need one par file to replace it, use SmartPAR or Mirror and it will fix the Jenna Jameson movie up for you real good. It's not limited to just .rar files either, you can par anything.