When you delete a file in Mac OS X (or even Windows for that matter), you are not actually erasing the file, but rather removing the references to the file on hard drive. The data remains on the drive until it is overwritten by other data.
Removing files permanently (or shredding files) is required in several situations - to remove data that may incriminate you (BitTorrent downloads?), to remove data before selling a computer (most do not erase files permanently, leading to serious security concerns) etc.
Permanent Eraser is a free software for Mac OS X that removes files permanently. It uses the Gutmann Method (of overwriting files 35 times) and does various other things to obfuscate your data before removing it permanently. Once permanently erased, chances of recovering data are slim.
Get Permanent Eraser
