Fragmentation refers to the condition of a storage disk (a hard drive, for example) in which files are divided into multiple pieces spread across the disk.
Fragmentation occurs naturally the more a disk is used by creating, modifying, and deleting files on it.
There comes a time when the operating system needs to store parts of a file in physically non-contiguous sectors (clusters) on the storage disk. This is completely invisible to the user, but slows down the speed at which stored data can be accessed.
What happens is that the read-write head must search through different areas of the disk and group them together to treat them as a single file.
Defragment
Operating systems resolve fragmentation of files on a disk by using a defragmentation tool. Old systems required the user to run the application from time to time and thus optimize data storage on the disk, resulting in an increase in disk speed.
In the DOS 6.0 and higher system, the DEFRAG command was used.
In Windows, the Windows Defragmenter tool was also incorporated.
Currently Windows automatically takes care of keeping the hard drive up to date.
See: Defragmenter.
RAM Fragmentation
Fragmentation can also occur in RAM memories that are small, producing unused «holes» throughout. This is technically called external fragmentation.
With modern operating systems that employ paging schemes, a common type of RAM fragmentation is internal fragmentation. This occurs when memory is allocated in frames and the size of one frame is greater than the amount of memory required.
Why is a hard drive fragmented?
To understand exactly the mechanism by which files are fragmented within a hard drive, I suggest reading the article: hard drive.
related terminology
Defragment
Defragmenter
Disk sector or cluster
File System
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