Suppose I want to erase all the data on my hard disk, then I boot using a LiveCD like Knoppix and open a shell and type the following command:
shred -vfz -n 10 /dev/hda
Here /dev/hda is my whole hard disk. And I am asking shred to make (-n) 10 passes by overwriting the entire hard disk with (-z) zeros. And shred program (-f) forces the write by changing the permissions wherever necessary.
10 passes is probably way overkill, I'm not aware of a single confirmed example of someone recovering data from an erased disk even after 1 pass. Its a time-paranoia trade off.