Just as on Windows - when you delete a file the space is marked as 'available', but the information isn't necessarily actually removed from the storage medium. The trash (or recycle bin in Windows terms) is the OS remembering a certain amount of the storage location data and still reserving that space for that previous file. When you empty the trash, that memory of those storage locations is removed but the files MAY still be accessible if you have the right tools, until such time as those storage locations are overwritten.