Let's say you have a tar archive that contains the following.

tar -tf example.tar
. . .
tmp/sample/
tmp/sample/bar.txt
tmp/sample/foo.txt

The tar command with the -x or --extract and -f or --file optiions will extract the contents of the tar archive. In this example, the contents of example.tar will be extracted into the present working directory.

tar -xf example.tar

The -v or --verbose option can be used to display the files being extracted.

tar -xvf example.tar
. . .
tmp/sample/
tmp/sample/bar.txt
tmp/sample/foo.txt

As an example, if your present working directory is /home/john.doe, following will be where the archive is extracted.

/home/john.doe/tmp/sample/
/home/john.doe/tmp/sample/bar.txt
/home/john.doe/tmp/sample/foo.txt

The  -C or --directory options can be used to extract the contents to a specific directroy. In this example, both of these commands have the same result, where the contents of example.tar are extracted to the / directory.

tar -xf example.tar -C /
cd /; tar -cf example.tar

The -z option will be needed if the archive is gzip compressed.

tar -zxf example.tar.gz

The -j option will be needed if the archive is bz2 compressed.

tar -jxf example.tar.bz2

To extract a single file in the archive, include the path to the file in the archive.

tar -xf example.tar path/to/example.txt