.tar files are known as 'tarballs'. They are 'Tape ARchives' - they contain files, but do not compress them, unless run through 'gzip' or 'bzip' after generation.

To extract from a .tar: tar xvf file.tar
[x extracts, v makes verbose, f informs tar that filename follows]

To decompress and extract from a .tar.gz: tar zxvf file.tar.gz
[z specifies gZip]

To decompress and extract from a .tar.bz2: tar jxvf file.tar.bz2
[j specifies bzip]

More information: man tar