Simply erasing a hard drive does not destroy the data on it. The computer's erase just wipes out the directory. The data is still there and easy to recover.
If you are selling or giving away a hard drive, FORMAT IT, preferably more than once. Personally, I would physically destroy my old hard drives rather than sell or give them away.
If you are selling or giving away a computer, you need to repeatedly over-write the files using tools which can be found online. Even this can be risky since modern computers are designed to leave "forensics files" hidden in places where the users are not likely to look containing information of potential interest to law enforcement. When I process a computer for "Hawaii's Computers For Kids" I will mil-spec wipe the drive completely, then reinstall a new operating system. These days I put LINUX and Open Office on the machines since Microsoft is being such pricks about donating XP to charities.