Fragmentation is caused when an operating system breaks a file into pieces because there is not enough space on the storage device where the file was originally saved.

Defragmentation is the term given to the process of scanning the file system and rejoining the split files back into consecutive pieces.

Fragmentation Fragmentation commonly occurs when old files are opened, modified and subsequently saved. One example of this, would be where a previously saved file, let's say, a document, is opened and added to. This will cause the file to be larger in physical space than when it was first saved. The operating system will then break the file into 2 or more pieces, and store those pieces (fragments) in different parts of the storage area. The file system, such as File Allocation Table (FAT) or NTFS, would then keep a record of where the different fragments of the file are stored.
When the operating system requires the file again, it will query the file system (FAT/NTFS/or other) to find out where the different fragments of the file are located on the partition (drive).
Defragmentation As noted above, the process of defragmentation rejoins the fragmented parts of a file. It loads the file fragments and then saves them in consecutive parts of the storage. The process of defragmenting can be time consuming, but it is one of the easiest ways to increase the performance of your computer. The frequency at which a PC should be defragmented will directly depend on the amount of usage area. For more details about how to defrag your drive, see our guide to Hard Drive Maintenance for Windows 98 and XP.

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