How it get bad sector on hard drive

Bad sectors can be brought on by numerous causes such as the following:

Physical damage
Degredation of the HDD over long periods of time/high usage
High areas of corruption due to data.
NTFS/fat32 (file system failure)

Some of these can be recovered such as minor file system failures and data corruption though its very rare that physical damage of the disc would be recovered.

Thanks
Matt