A hard disk drive (HDD, or also hard drive) is a fixed data storage device that stores data on a magnetic surface layered onto hard disk platters.
A hard disk uses rigid rotating platters (disks). Each platter has a planar magnetic surface on which digital data may be stored. Information is written to the disk by transmitting an electromagnetic flux through an antenna or read-write head that is very close to a magnetic material, which in turn changes its polarization due to the flux.

The information can be read by a read-write head which senses electrical change as the magnetic fields pass by in close proximity as the platter rotates.

Disk failure occurs when a hard disk drive no longer operates and the information on it can no longer be accessed by the computer.

This can happen for no reason at all or due to an external factor such as exposure to fire or water or High Magneticwaves or suffering a sharp impact
How seriously the disk failure is varies.

The most serious and well-known kind is the head crash where the internal read-and-write head of the device touches a platter or a magnetic data storage surface. A head crash normally causes severe data loss and, moreover, data recovery attempts may cause further damage if not done by somebody with specialized knowledge.

There are also controller electronics which occasionally fail. If that happens it may be possible to recover all data.

When your computer's hard disk starts to act funny, make sure that you have an up-to-date backup. Then you can prepare some simple diagnostics and possibly repairs. Both Windows and Mac OS come with built-in hard-disk utility software that scans your hard disk for errors and attempts to fix them.

This is what you can do for Windows Steps: 1. Double-click on My Computer to open the My Computer window. 2. Select the disk that you want to diagnose and repair. 3. Choose Properties from the File menu. You can see the Properties window for the drive that you selected. 4. Choose the Tools button. 5. Click the Check Now button under Error Checking Status. 6. Choose either "Thorough" or "Scan for and Attempt Recovery of Bad Sectors." 7. Click on Start.

Tools to recover data are various such as:

There is a Data Recovery Utility which examines your inaccessible hard drive for damages and corruptions and recovers the data back.

Hard drive can be recovered from Windows 9x / ME / NT / 2000 XP / 2003 disk recovery
Supports FAT16, FAT32, NTFS, NTFS5 file system
Provides data recovery from Formatted logical drives
Recovers data from Re-partitioned & Missing/ Lost Logical drives
Lost folder and deleted file recovery.

There is also a partition recovery utility available that helps you in recovering all your important data lost after accidental formatting, virus problems, software malfunction,and file or directory deletion. There are also easy to use FAT & NTFS Partition File recovery Utilities that examine your inaccessible hard drives for damages and corruptions and recover the data.

There are also systems which:

Provide Partition recovery from FAT16, FAT32, NTFS, NTFS5 file systems.
Recover deleted files/folders even after recycle bin has been emptied or use of Shift+Del key.
File recovery from deleted partition, lost partition missing partition or formatted logical drives.
File recovery from Missing or Lost folders.
Recognize and preserve long file names when restoring files & folders.
Multi-Disk Drive Support - Performs NTFS recovery on all IDE, EIDE and SCSI disk devices.

There is also software that recovers corrupt or lost data from floppy disks.

The software supports inaccessible floppy disk data retrieval, FAT, BOOT or ROOT area damaged situations, and rescues various corrupt files from a diskette. BadCopy Pro can recover data from floppy disks that Microsoft Windows identifies as "not formatted", "not accessible", or prompts you to format.

The software does not write data to your floppy disks, but saves the recovered data to a new file that you specify.
It can fix corrupted files on floppy disk.
It can restore damage to a FAT (File Allocation Table) or BOOT area of the disk.
It can repair damage to the ROOT area of the disk, so that files can be listed again.
It can erase Viruses on a diskette.

There is also software available that fixes corrupted or damaged Microsoft Word documents.

There is also a new product on the market; software that creates a Windows CD for you, and creates recovery files so you will not need to format to reinstall Windows.

It has been asked many times "I have Windows XP (or ME), how can I install Windows without having to lose all my files. I only have a Recovery Disk". Well it is really very simple, so long as you have a CD burner; or at least a second hard drive.