In the context of administering computer systems, defragmentation is a process that reduces the amount of fragmentation in file systems. It does this by physically organizing the contents of the disk to store the pieces of each file close together and contiguously. It also attempts to create larger regions of free space using compaction to impede the return of fragmentation. Some defragmenters also try to keep smaller files within a single directory together, as they are often accessed in sequence
[edit] Approach and defragmenters by file system type
FAT: MS-DOS 6.x and Windows 9x-systems come with a defragmentation utility called Defrag. The DOS version is a limited version of Norton SpeedDisk.[10] The version that came with Windows 9x was licensed from Symantec Corporation, and the version that came with Windows 2000 and above (with the exceptions of Vista, Server 2008, and 7) is licensed from Diskeeper Corporation.
NTFS: Windows 2000 and newer include a defragmentation tool base ...more

Fragmented Datas/files are brought in order to save memory space and enhance speed of processing.

Process that reduces the amount of fragmentation in file systems.