The old definition of 'low-level format' meant completely rewriting the track and sector markings on the disc. The drive heads moved in fixed steps, and after time or a lot of use, the step positions of the heads may drift away from the magnetic markings.
For about 10 years, this this type of format has been impossible; the tracks and sectors are so closely packed that the only way to seek at all is to use the track/sector markings as a guide. Drives which cannot be formatted in this way ignore the BIOS format command.
Because of this, 'low-level format' has now come to mean something different: it now means deleting information about any partitions and any OS specific information, so that the drive appears totally blank to your OS.
All drive manufacturers offer utilities that do this, although they aren't usually called 'Low-level format', instead they are often called 'Wipe' or 'ZeroFill' or something similar.
LLF will delete all OS specific data on your drive - this includes the Windows/DOS bad sector list. Normally, Windows/DOS will preserve this list when you format, so as to protect your data from being stored in bad sectors. The format itself does nothing to the actual sectors themselves, if they really are bad, then they stay bad - except that now, Windows/DOS doesn't know about them, and will happily save your files in them.
