Well the data wont stay on your HD forever if you delete a file , but the process just deletes its entry in the address table.
This and a number of other programs can recover them, but in time those data areas will get overwritten with new files.
Of course the Recycle Bin stuff can be restored quite easily, but if you emptied it and needed to recover you could use this program.
If the files were lost due to a disc defect or virus it may or may not recover them.
Remember Backing up important files (CD or usb hard drive) is safer.