A file extension of .tar simply means that the file has been archived with the GNU tar application. It is only for human reference as linux does not give a crap what extension a file has. If you want to build something from source it is probably in .tar.gz or .tar.bz2 form. You can uncompress them like so:

tar xzvf file.tar.gz

OR

tar xjvf file.tar.bz2

NOTE that file.tar.gz is the same as file.tgz

After you uncompress one of these archives it will create a new directory usually named similarly to the file you just uncompressed. Simply do cd file and read the README or INSTALL documentation to find out how to install the application.