The most common "data recovery" scenario involves an operating system (OS) failure (typically on a single-disk, single-partition, single-OS system), in which case the goal is simply to copy all wanted files to another disk.
Another scenario involves a disk-level failure, such as a compromised file system or disk partition, or a hard disk failure. In any of these cases, the data cannot be easily read. Depending on the situation, solutions involve repairing the file system, partition table or master boot record, or hard disk recovery techniques ranging from software-based recovery of corrupted data, hardware-software based recovery of damaged service areas (also known as the hard drive's "firmware"), to hardware replacement on a physically damaged disk. If hard disk recovery is necessary, the disk itself has typically failed permanently, and the focus is rather on a one-time recovery, salvaging whatever data can be read.
In a third scenario, files have been "deleted" from a storage medium. Typically, the contents of deleted files are not removed immediately from the drive; instead, references to them in the directory structure are removed, and the space they occupy is made available for later overwriting. In the meantime, the original file contents remain, often in a number of disconnected fragments, and may be recoverable.