Suppose you want to erase all data on a hard-drive before selling or trowing it away.
Use the following commands to do so.
# sudo fdisk -l
to Identify the hard-drive to be deleted
# shred -vfz -n 10 /dev/sdb
(where sdb correspondence to the devicename you identified via fdisk)
This will overwrite the hard-drive 10 times and should be enough for standard hard-drive deletion.