The file is stored twice. It is tarred, which combines all the files together, then it is gzipped.
file -> file.tar -> file.tar.gz.
tar file -> file.tar
gzip file.tar -> file.tar.gz
gunzip file.tar.gz -> file.tar
untar file.tar -> file
Read the manpage for tar, it tells you how to automatically ungzip it using the -z parameter. It goes on to explain how this .tar and .gz business works, which is helpful no matter which OS you're on.
Also, the word is "why", not "y".. I spend a lot of time helping you here, don't be lazy. It makes me feel like I'm wasting my time.