From day to day there are articles in the news about people who got a used computer with sensible private data on it. Even if the hard drive was formatted it’s quite easy for skilled people to take a look at the data which was stored on this drive. To safely erase the data it makes sense to overwrite the disc with new data which make it quite impossible to access the original data.
An easy way to do this is under Linux the command mkfs.ext3:
Replace drive with your hard disk (i.e. /dev/sda1) and the partition is erased.