Erasing and Formatting
Simply erasing all the data on your hard drive and formatting it is not enough security. You can spend hours going through your hard drive and deleting all the files and documents you want, but using the delete key on your keyboard in Windows will only remove the shortcuts to the files making them invisible to users. Deleted files still reside on the hard drive and a quick Google search will show many options for system recovery software will allow anyone to reinstate that data.
Formatting the hard drive is a bit more secure than simply erasing the files. Formatting a disk does not erase the data on the disk, only the address tables. It makes it much more difficult to recover the files. However a computer specialist would be able to recover most or all the data that was on the disk before the reformat.
For those who accidentally reformat a hard disk, being able to recover most or all the data that was on the disk is a good thing. However, if you're preparing a system for retirement to charity or any other organization, this obviously makes you more vulnerable to data theft.