Answer Description
When data is deleted or a disk is formatted the location of the data is tagged as empty and available for use but is not actually physically removed from the disk. This means after deleting it is still possible to read the data. Overwriting is a method that deletes the data and overwrites the physical location with random information to ensure it can never be recovered.
Wikipedia
Data erasure (sometimes referred to as data clearing, data wiping, or data destruction) is a software-based method of overwriting the data that aims to completely destroy all electronic data residing on a hard disk drive or other digital media by using zeros and ones to overwrite data onto all sectors of the device By overwriting the data on the storage device, the data is rendered irrecoverable and achieves data sanitization Ideally, software designed for data erasure should: Allow for selection of a specific standard, based on unique needs, and Verify the overwriting method has been successful and removed data across the entire device
Data_erasure - Wikipedia, the free encyclopedia