Is it really deleted?

Think of your hard drive as a whiteboard. Think of your operating system as the teacher at the front of the class. Imagine the teacher is writing continually, frantically, all over the whiteboard as the lecture proceeds. He doesn't erase the board every time there's something new to scrawl on it, he looks for an empty spot first. Only when there are no empty spots remaining does the eraser come out. This is how data is placed on your hard drive.
And at the risk of carrying the analogy too far: the eraser might need to be cleaned. So when the teacher uses it, you actually get less than an optimal erase - you get more of a 'smear'. And although the students can read what's being written this time around, they can also see traces of what used to be there before. This is an illustration of how your computer's disk is used to store files.
The original question actually falls on its own absurdity. For an operating system to really erase deleted files it would have to spend just as much time 'erasing' those files as you needed to write them in the first place. This is neither user-friendly nor productive.
What the system will do when you delete a file is 'unlink' them. Meaning you won't be able to find the deleted files anymore (but others might). Most systems have only one 'link' per file; Windows systems only have one link per file; when you unlink ('delete') a file on a Windows system, it's no longer accessible by you as a user.
But the file itself remains. And most often, the actual link to the file is still there as well - and only minimally adjusted so the system knows it's not to regard the link as a link anymore.
A file with a name like 'SECRET_FILE.TXT' might - after so-called deletion - be renamed '?ECRET_FILE.TXT' and the '?" in the beginning will tell the system that the file is to be regarded as deleted. In this way, deleted files are certainly not deleted. The file itself remains intact for the time being on your hard drive and there are many programs out there that can fully recover it.

Article Source: Christine Ellis