(the info below is an attempt to explain data storage on a PC in non-overly-technical terms)
-History of websites you visited.
-Emails you sent and received, especially if you use an email program such as Outlook, Windows Live, and others.
Why should you want the above info deleted?
-It may be private / personal.
-Keep it out of the hands of a potential identity thief, or worse.
How is the data stored?
On magnetized metal plates. Those plates are your hard drive.
Each ring contains information that was written by magnets.
There are special utilities that actually write over (overwrite) the area that data used to occupy, which is a more secure way of erasing it.
Even when the rings are erased, sophisticated programs may be able to recover some or all of the info by searching the magnetic fields on the sides of the rings.
How should you delete your confidential data?
Speak to a friend, relative, neighbor etc. who can actually come over and take care of it for you.
FYI, data on magnetic media can be destroyed by extremely strong magnets, high heat (like an oven), or physically destroying the media.