When simply formating a drive, only the file catalog is erased. The contents of the disk are still there and can be recovered with special software. Zeroing the disk means writing zeros to the whole disk. In this process, malfunctioning sectors of the disk are mapped out and no longer used. Can be helpful for fixing bad drives or completely erasing the contents. To be completely sure that data cannot be recovered, the disk must be zeroed between 8 and 30 times.
The directory will be emptied and zeros will be written to every data block on the drive. If you want more security and you are using Panther or Tiger you can use the eight way write which overwrites every data block eight times and erases the drive content to a level that it is highly unlikely it could be recovered even by the National Security Agency or FBI labs.
writing to zero's is like getting a brand new un used hard drive. in effect
a new hard drive has millions of sectors. when data is added, these sectors change to the relevant data, and is made 'ledgible' by the computer.
erasing/formatting a drive doesn't change the data much, just makes the computer 'think' a new, clean hard drive is installed. the data is still there, but gets written over when needed (if you lose a lot of data on a drive, and you are considering data retrieval DON'T USE THE DRIVE. it'll start writing over, quickly, as soon as it uses the disk for anything, thus losing the retrievable binary)
writing it all to zeroes acually goes through every sector, like a bulldozer, making sure every bit of data is over written by a 'nothing'. multiple passes only offer further security.
It takes bloody ages. like about 2 days for a decent hard drive at 16x passes