Yes, some ISPs offer usenet for free. However, in general they are poorly maintained with short retention for binaries (if binaries are even offered) - usually just a few days. This makes downloading files much harder.
Pay services have much greater retention, some for as much as three months with a 99.9% retention rate, meaning all parts of the file are present and have not be accidently deleted early.
RAR is the popular format for files on usenet. It is a compression method, also it allows segments of a file to be spread across multiple post. WinRAR is the program used to decompress the content into a useable format.
PAR files are "check" files that can tell you if you have the entire post, and if any part of it is damaged. If it is they also function as repair files that can correct you download or make up missing pieces. You are correct that QuickPar is the program to use with PAR files.
