When you delete a file it is not immediately removed from the hard drive. Only it's place in the File Allocation Table (FAT) is removed. The FAT is similar to a Table of Contents in an extremely large book. If you removed an item from it you would have a very hard time finding it in the book. Deleting a file and removing it from the FAT informs Windows that this space is available for overwriting. It will be overwritten the next time you copy something that is the same size or smaller.
This could be a few minutes or much longer if what you are copying is larger. At any time before that, the file can be recovered using any Undelete program. Shredder overwrites the file you choose with binary data (basically garbage) making it unreadable and useless even if it is recovered. It will work with any type of file and it's use is very basic and simple. This version is the first release on CNET Download.com.