"tar" is not a compression format. tar is an old standard UNIX archiving format. It actually comes from "Tape ARchive". You will need a PC version of tar to un-tar it or as Lusifer said Winrar, which apparently understands the format. tar actually does padding when it archives, so it offers no compression, but certain UNIX compression programs such as "compress" or "gzip" can be used on a tar archive to achieve better compression rates than you typically find with things such as zip files. Those who understand UNIX might be interested in the Cygwin toolkit at
http://www.cygwin.com. It's free and has just about every UNIX program you'd want ported to Windoze.