Disk recovery are two words one never hopes to have to utter during their IT career. The process is ultimately very time-consuming and quite costly, plus it's also almost completely avoidable by implementing a solid, scheduled backup solution and taking some care to follow safe computing habits and best practices.
However, in the event of a catastrophic failure that results in a corrupt or failed drive, all is not lost just yet. Thankfully (for end users), data recovery takes on several levels due to the nature of failure in the hard disk. For many individuals, the first cause is also the most common: deleted or "lost" files. This is followed by the next level of failure, which typically occurs due to OS-level system file corruption or the occasional malware infection. These render the drive physically operable, yet the file structures or tables have been rewritten or deleted altogether, so the files unable to be located — almost like a book with a table of contents.
The last level is by far the worst and equally most expensive: hardware failure. This can be anything from clicking noises (aka "the click of death") to grinding noises or even the loss of power to the drive. In most cases, by the time the disk reaches this level, there's little to be done but accept the loss or take it to a forensic lab for scientific recovery and/or repair.
This article will deal specifically with levels one and two, since they're the most common and the easiest to recover data from. In many cases, the data may even be recoverable by the end user (or system admin) with a specialized recovery software application.