The Windows OS, MAC OS, Android OS and iPhone OS can all be considered as libraries.
Libraries in the physical world do not have infinite shelf space.
Therefore, a system was needed to track where books should be placed on shelves, the most commonly known system being referred to as the "Dewey Decimal System".
An "inode" value for a folder or file can be thought of as a Dewey Decimal value that each book in a library is assigned. The "librarian" of the Windows, MAC, Android, etc. operating systems track folders and files stored within the shelf space of each computer or smartphone by assigning each folder or file a unique inode value just as a real life librarian would assign a book a unique Dewey Decimal value.
For example, Windows computers use inode values in the Master File Table (MFT) to track files and folders.
If an end user moves a file to the Recycle Bin on a Windows computer, the inode value for that file is still tracked by the Windows' card catalogue system until such time as an end user empties the Recycle bin.
When a Recycle bin is emptied on a Windows system, the Windows librarian no longer tracks the inode value for the "permanently" deleted file (yes these permanently deleted inode entries can be carved if not overwritten at a later point by tools like TZWorks' excellent suite.), with the space that the now "permanently" deleted inode entry, or card in the card catalogue system, now becoming unallocated space within the MFT.
My real world analogy is a librarian placing books to be discarded in a real trash bin and crossing out the card catalogue entry for those discarded books. At this stage, one could still take the books out of the trash bin and use the card catalogue to place the book back on the shelf.
Emptying the real life trash bin of library books (subsequently taken away forever by the garbage men", and then tearing up and permanently removing the discarded books' card catalogue entries, would be permanent deletion in the real world.
On a separate note, my forensic research is showing that, whereas Windows and MAC OSX head librarians work for me, my Android and iPhone OS librarians ultimately work for Google and Apple in regards to the fact that all of my Windows and MAC book shelves are accessible by me, but unless I root or jailbreak a phone, there are book shelves in my Android and iPhone libraries that only the librarians reporting to Google and Apple can access. Hopefully this analogy is making sense, but this concept of whom one's device's librarian ultimately reports to is the root cause (pun intended) why computer hard drives can be physically imaged (all book shelves copied) whereas non rooted/jailbroken iPhone or Android phones can only be logically imaged.
Both smartphones and computers use inodes as the unique "Dewey Decimal" value to track folders and files in their respective libraries, but not all shelves of books are accessible to the end user or forensic expert as purchased from the factory.
From Source http://en.wikipedia.org/wiki/Dewey_Decimal_Classification:
The Dewey Decimal Classification (DDC), or Dewey Decimal System, is a proprietary library classification system first published in the United States by Melvil Dewey in 1876.[1]
The Decimal Classification introduced the concepts of relative location and relative index which allow new books to be added to a library in their appropriate location based on subject. Libraries previously had given books permanent shelf locations that were related to the order of acquisition rather than topic. The classification's notation makes use of three-digit Arabic numerals for main classes, with fractional decimals allowing expansion for further detail. A library assigns a classification number that unambiguously locates a particular volume in a position relative to other books in the library based on its subject matter. This makes it possible to find any particular book using the number, and to return it to its proper place on the library shelves.[notes 1] The classification system is used in 200,000 libraries in at least 135 countries.[2][3]
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