Untar

if it ends in .tar:

tar -xvf filename.tar
(you don't need the v, it is for verbose, i like verbose though)

if it ends in .tar.gz:
tar -zxvf filename.tar.gz

if it ends in .tar.bz2:
tar -Ixvf filename.tar.bz2 or tar -jxvf filename.tar.bz2

(many non debian systems don't have bzip2 integrated into
their tar yet.. debian 2.2 uses -Ixvf and debian 3.0
uses -jxvf)

if it ends in .tar.Z:
tar -Zxvf filename.tar.Z