Computers have many components that allow them to function, each part has its own use and each contributes to the way the machine works. One of the most vital parts of the system is its Hard Drive, without one the machine will not work (although the same goes for most other components as well). The Hard Drive is a storage medium that is used to install the operating system (Windows, Linux or Mac), the installation of software i.e. game, work programs, email clients etc are also installed on to the HD. The other items it is capable of storing is general data (word documents, excel spreadsheets and databases among others), these can be very important documents to the user and contain sensitive, vital, confidential or personal information.
When storing information that you know you need, it is common practice to back up your hard drive to a different media (an external drive or CD).
If you have done this and your HD was to fail, your information would still be safe and accessible from a different location, however if you have not done this and your drive fails, do not panic, you have not necessarily lost everything. There are companies out there that specialize in data recovery: they employ skilled individuals that are capable of retrieving data from a faulty Hard Drive.
This is done by recovering bad sectors, sometimes the drive will have to be taken apart and the damaged parts replaced for parts of a working drive. Even if your hard drive has completely burnt out there is still a chance the can retrieve your valuable data.