When you delete files using the Finder, the deleted files are placed in the Trash folder. You can easily recover deleted files from the Trash folder. When you choose to empty the trash, there is no way to recover the deleted files from the Trash folder. This gives the perception that the files are permanently gone and not recoverable.
However the content of the deleted items are still lying around on the disk until they are overwritten by other data. And with low level tools, someone could potentially recover part, most or all of the deleted content.
Many Mac users do not realise that there is an option to remove items in Trash securely under OS X. When this option is turned on, the Finder will overwrite on the disk random data onto the file content, making recovery using low level tools impossible.
The secure option takes a couple more computing cycles but is a worthwhile option if you’re working on sensitive confidential data. To enable empty trash securely, open Finder Preferences. Select the “Advanced” tab, and check the option “Empty Trash securely”.