When you delete files from a hard disk, the storage space that the files occupied is available for use. However, the deleted files can still be recovered using data-recovery software.
If you've installed Mac OS X El Capitan, you may have discovered that the Secure Empty Trash feature is missing and it doesn't seem as if you can get it back.
A support page on the security content of El Capitan says: "An issue existed in guaranteeing secure deletion of Trash files on some systems, such as those with flash storage. This issue was addressed by removing the Secure Empty Trash option."
Previously, if you wanted to delete files that contain sensitive information, you could use Secure Empty Trash. Files deleted in that way were completely overwritten by meaningless data.