~$ tar cjvf foo.tar.bz2 bar
The above command will create a tarball foo.tar.bz2 containing bar. bar can be a single file or a folder. If it's a folder, tar will recursively add any files and folders it finds inside.
Here's what the options mean (as written in the manpage):
create a new archive
-f, --file [HOSTNAME:]F
use archive file or device F (otherwise value of TAPE environ‐
ment variable; if unset, "-", meaning stdin/stdout)
filter archive through bzip2, use to decompress .bz2 files.
WARNING: some previous versions of tar used option -I to filter
through bzip2. When writing scripts, use --bzip2 instead of -j
so that both older and newer tar versions will work.
verbosely list files processed
If you prefer gzip then replace j with z. You can also add more than one file or folder simultaneously easily.
~$ tar cjvf foo.tar.bz2 bar1 bar2 bar3
For those not comfortable with the terminal, Ubuntu's archives feature a few backup tools like Simple Backup Restore, Deja Dup Backup and File Backup Manager although I haven't tried any of those.
With the release of Karmic, it's a good thing to do a backup of your personal files whether you plan to upgrade or reformat.
A restore is equally simple (x means extract):
~$ tar xvjf foo.tar.bz2
or, for gzip archives:
~$ tar zxvf foo.tar.gz