tar is a UNIX utility to create archives (or tar balls) from directory hierarchies. Say that you want to send someone an e-mail with the contain of a directory and of all its subdirectories. tar allows you to bundle the whole thing together into a tar ball. Upon reception of the tar ball, the recipient can unbundle it to reconstruct the whole directory structure.
Example:
% tar -cf archive.tar dir
creates a tar ball (archive.tar) of the content of directory dir.% tar -xf archive.tar
untar the tar ball archive.tar and creates the directory dir.
See the tar man page for more details (type "man tar" at the UNIX prompt).