What is the difference between "Size" and "Size on disk" when we see the properties of any file or directory.

"size" refers to the number of characters (ie "bytes") that are actually within the files' contents; for example, if I create a very small NOTEPAD (i.e. text) file that contains the word HI, then the "size" is 2 bytes (or 4 bytes, if you press the ENTER key after the word HI when you create the file -- since every line of text contains 2 special "invisible" characters [CR and LF] at their end whenever the ENTER key is pressed). However, that 2-byte (or 4-byte) file can only be saved in the "smallest unit of storage" (known as a "sector"); therefore, with a normal/most-popular disk-sector-size of 1024 bytes, that 2 or 4-byte file will show a "size on disk" of 1024 bytes (aka "1 KB") the smallest unit of storage on most commonly-used computer disks (some larger-scale computers use large-scale disks which have a different sector size -- but a disk always has a FIXED-SIZE "sector" which is pre-determined before you start using your disk). An easy way to think of the disk is to picture your disk as being a bunch of fixed-size empty boxes (or empty 1-dozen egg cartons) -- that is, any FIXED-SIZE empty containers, because the computer manages fixed-size amounts much more efficiently than variable-size containers -- the same way people packing boxes would be more efficient in an assembly line if ALL OF THE BOXES they had to pack were always the same size, so they would not have to worry about "what is the next size of box" that will come along and cause me to wonder about how to pack it (that is, it eliminates pausing, thinking and confusion). That means, whenever you store a file on disk, it gets spread-out into as many boxes or partial boxes as it needs, BUT it never shares its boxes (ie "sectors") between files; again, you would picture the analogy I'm using as putting various objects into boxes (each different object being a different file); for example, a bunch of APPLES could be 1 file, a bunch of ORANGES are another file -- an since the bunches may be different sizes (BECAUSE ALL FILES ARE USUALLY DIFFERENT SIZES, maybe more oranges than apples), the ORANGES may require 13 boxes (with the 13th box being only half full) and the APPLES may require 7 boxes (with the 7th box being only a fourth full) -- that's exactly the way files (large or small) are put on disk, the disk uses as many "sectors" (e.g. boxes) as it needs to store a "file" (e.g. ORANGES). Therefore, the next time you store a file on disk, just ask yourself this simple question -- which will help you to easily remember how a disk works: "I wonder how many FIXED-SIZE boxes [sectors] are going to be needed this time to store my ORANGES [file]?". The concept is easy, as is the case with almost all technology; the only 2 things that become confusing are: trying to understand "the terminology"; and, "how the computer keeps track of all its BOXES" -- and those are easy too if you expand your vocabulary and use analogies/examples similar to what was done above (with ORANGES and BOXES).

Size is nothing but the actual size of the file. But size on disk is the size it has occupied on the disk. We can't save one bit of information on hard disk. We can save a minimum of one byte on the disk. But it will take the fragmented space also in consideration. So the the disk on size will be the total fragment space (including size of the file)

size is the complete, original, entirety of the file. size on disk is the file size after it has been compressed, and compacted to save space on the disk, it automatically gets rid of un-nesecary disk space, but none of the contents are removed.