With the command (GNU tar release)
tar ztvf yourfile.tar.gz
you'll find what's inside your gzipped tarball archive file.
If you want to uncompress all stuff that are into your gzipped tar archive you have to fire:
tar zxvf yourfile.tar.gz
All files will be untared into the current path from where your are calling the previous command.
If you just want to uncompress a given relative path from your archive into your current path you can use the command
tar xzvf yourfile.tar.gz path_inside_the_tarball_you_want_to_uncompress
It will untar only the files you want to uncompress. Those files will be released into your current working directory. If you want to release the files into another path you have to use the "-C" option followed by the path to the previous tar command.