A good way to think of it is like a book. All of your data is stored in "chapters" and their location is recorded in a "table of contents". When you delete something, the operating system erases the "chapter" or files location from the "table of contents". The chapter is still in the book, but it doesnt show it in the table of contents, or to the user. To actually erase the data though, you have to go and reformat the hard drive in which case the laser that reads and writes with will burn the layer of material smooth again and get out the notches that it created when you wrote data to the disk. The process is more complicated then that but for a general sense it just creates divots in the cd when you write to the disk and then smooths it back out when you erase the data.