Bad sectors are inaccessible or un-writeable portions of your hard disk and can happen on account of software malfunctions, physical damages on the surface, wear and tear or even defects in manufacturing. Small bad sectors are manageable but you should get the hard drive replaced in case there are bad sectors in larger numbers since these will eventually spur drive failure and loss of data. You should keep tracking the HDD status with suitable applications. Remember that even if the hard disk drive is working fine, a small bad sector cluster can eventually lead to data corruption and eventual loss. Whenever a bad sector is garnered by the system, the sector is marked and ignored. This will be remapped and files being read and written to this sector will be transferred somewhere else. You should regularly back up your data and consult professionals for recovering information from bad sectors.