tar xvf something.tar
If the tarball has also been gzipped (compressed), you can use the following command:
tar xvfz something.tar.gz
The biggest thing that I always forget when trying to use this command is that it doesn't accept the "-" prefix before its modifiers like most other linux commands.
If you need to create a tar file
tar -cvzf name.tgz subdirthe 'z' is optional and gzips the file.
the 'v' is optional and gives a verbose output.