Defrag Defrag Defrag. WTH is it?

Defrag and Defrag, all your IT friends IT admins always ask you to do it. But WTH is it?

Copy, Move, Cut, Paste, Delete, Install, Uninstall, Download.... So on and so forth.
While doing all this minor operations, your hard disk is actually quite messy, is just that you do not know.

Lets say you have a new hard disk. It clean[physical - internal]. So you start adding stuff. Then they take up some space from your hard disk. And then you delete some of them.

Result? Your hard disk is messed up. But why? I had arrange the files in folders. Yes they are arranged on the appearance but not physically.

To simplify the explanation I am going to use an analogy.
There are 2 empty drink bottles A and B[capacity of 3L each], you have 4 semi-liquid object, Q,W, E, R, of  1L, 1L, 2L, 3L respectively.

1. You added in Q and W bottle A.
2. You want to add R into bottle A but its already half full, so you split(fragments) it up into 1L and 2L and put the remaining 2L into Bottle B. Thus there is fragmentation.
3. You decided to remove Q from bottle A.
4. Then you add E into Bottle A but it was full and you split it(fragments) into 1L and 1L

So now the bottle A contain some of W, E, R.
Bottle B contain some of R.

When you want to get whole of E, you have to retrieve it from 2 bottles, thus wasting your time. This is where defragments comes in. Defragmentations, will actually move W and E into bottle A and the remaining W into bottle B. Thus keep all related files altogether.

So a hard disk also work quite similarly, thus as a hard disk get used too often, they tend to slow down, and you should start defrag-ing your hard disk soon.