Difference between thorough scandisk and defrag?

[AuraEdge]

I bet theres some HUGE DIFFERENCE where they have nothing to do with eachother, but hey, cant hurt to ask the experts.

[Dominus]

To the best of my knowledge, Scandisk just scans files/dirs/free space for errors and bad sectors, whereas Defrag actually moves and lines up the sectors on which files are written in order to reduce seek-time on your HDD, keeps the machine from looking all over the disk for parts of a particualr file.
Correct me if I'm wrong on anything here.

[Pushrod]

I agree with Dominus.

As you delete files or applications, it creats pockets of free spaces on your drive. As new files/apps are stored on your drive,some of these pockets may be to small for one file, hence that file will be broken in pieces and stored where there are spaces available.Defragmenting will recollect all separated pieces of a file and are put together, making file retrieval more efficient (faster and less work) and at the same time puts all free spaces together so that subsequent data or apps installations will also not be fragmented until you or windows delete some files again and the cycle goes on.