Another analogy on the recovery data is well represented by a book . Every file on your computer is a single page of a book and when you delete a file from your hard disk, does not mean that that page is torn and thrown away, but that is only hidden from reading. This means that page will disappear only from the summary of the book.
What a data recovery software does is getting back to the original file and restore it. The index, the "summary" of the hard disk is analyzed in order to find the missing file. But the longer you wait, the more difficult to recover the data and when too much time passes, even the most advanced data recovery software can have trouble finding 100% of the lost files . This is due to the fact that the operating system keeps the deleted files in a reserved space, which has a limited storage capacity. When this space is full, the system begins overwriting the old deleted files with new deleted files. Therefore, a file that has been accidentally deleted yesterday has many more chances to be retrieved compared to a file deleted 5 months ago.
Certainly, every data recovery software works in its own way and has its own methods for accessing those lost files and rebuild the original structure of the file. However, all are based on these basic principles and understanding of this is the most important step for a complete and efficient data retrieval.