OS X Mountain Lion: Prevent deleted files from being read
When you delete files, the disk space that the files occupied is available for use. However, until that section of the disk is overwritten with new data, the information from the file is still on the disk and can be retrieved by programs that read raw data.
If you want to delete files that contain sensitive information, you can place them in the Trash, and then use Secure Empty Trash to overwrite the information so it is completely removed.
Using Secure Empty Trash takes a bit longer than simply emptying the Trash.
If you are using disk encryption, you can set it to automatically overwrite your deleted files.
If you want to erase a hard disk and be sure no information is left, you can use one of the secure erase options in Disk Utilities. For more information on using secure erase, see Disk Utility Help.