noblues
A very simple explanation (I hope!)
defrag simply gathers all the bits of a file (which can be in several blocks all over the hard drive) and compiles them into one sequental block. No data is deleted.
Format puts markers for start and end of sectors on the hard drive, any data between these marks is left intact.
Fdisk does a similar thing.
The operating system uses a File Allocation Table (FAT) to store and retrieve files from the hard drive. Deleteing a file removes the entry from the FAT but leaves the data intact.
Windows explorer and the like display information taken from the FAT not from the disk surface.
From this you can see that if you can read the disk surface directly you can recover data that was "deleted".
Hope this helps
CK