to break it down:
file is the file name - doesn't tell us much
tar - this is a tar archive ("man tar" in a terminal window for more info)
bz2 - this file uses bzip2 compression (kinda like a zip file in windows or a stuffit file on a mac)
fyi: the shorthand version of this same file would be file.tbz2. That said, the extension doesn't matter with most linux apps, so it could just as easily be file.bkRulz & all the following would still apply.
to unarchive/uncompress this file, open a terminal window & do the following (in a terminal):
this will uncompress the bzip2 & leave you with a tar archive called "file.tar"
to unarchive the tar, in the terminal:
tar xvf file.tar
-- or --
you can do it all at once by telling tar this is a bzip2 archive, like so:
tar xvjpf file.tar.bz2
for more info, read "man tar".
if you're unsure of any file, in a terminal, type
and you can see what type of file you're dealing with.
edit: I knew someone would be me to the answer, but after all this typing, I'm leaving it here.