The Wikipedia dictionary explains it like this: "A bad sector is a sector on a computer's disk drive or flash memory that cannot be used due to permanent damage (or an OS inability to successfully access it), such as physical damage to the disk surface (or sometimes sectors being stuck in a magnetic or digital state that cannot be reversed) or failed flash memory transistors.It is usually detected by a disk utility software such as CHKDSK or SCANDISK on Microsoft systems."
In other word, the data saved on the bad sector area also lost as the bad sector is generated. Therefore, we should often run the CHKDSK tool to check our drives.
Yeah! As rdc85 writes, the bad sectors are irrecoverable. Therefore, for the data on our drive, we should use our drive rightly and properly, such as removing it with the "Safe Remove", doing regular anti-virus scan and clean the trash files regularly, etc.
Nearly forget! We also should keep at least two copies of everything important on a separate drive.