Hard disk data recovery has already been dealt in simple words. Let us discuss in little detail here. The data you store on your hard disk can be affected by different means. Even it happens sometimes that you accidentally delete the data which is highly important for you. The sources of data loss can also be some natural phenomenon where your entire hardware / hard disk gets damaged physically. There are also chances that you lose your data due to inappropriate storage of media. In all such cases, data recovery comes into picture and you can recover the entire data or at least most of that.

Data recovery is the process of extracting deleted / lost / affected / formatted / overwritten data from your storage media. In most cases, it is tough to recover the data from overwritten media. Your media may be hard disk, optical media like CD / DVD, floppy disks, portable media like flash drive or portable hard disks or any other kind of media.

Data recovery is done in different steps. It may include physical replacing of parts. This is usually done when your hard disk (mostly) is damaged physically or electrically. In such cases, replacing the damaged part can be enough to recover the entire data and make the disk re usable. In some other cases where above technique is not feasible, we go through advanced imaging techniques where the affected disk is imaged to another safe media and analysed for logical damage and recovered.

Read more on wiki about data recovery.