If you have a *.tar archive, you use $ tar xvf *.tar (the '$' is not part of the command, it just shows that you should type this in the command line, the '*' is the name of your archive)
*.tar files aren't compressed really, they are just some files packeged into one
If you have a *.tar.gz, you should use $ tar xzvf *.tar.gz
*.tar.gz archives are compressed with gzip
If you have a *.tar.bz2 you should first do $ bunzip2 *.tar.bz2
Then you will have a *.tar file, which one can be untar'ed via $ tar xvf *.tar
*.tar.bz2 are compressed with bunzip2
I hope I helped.