Defragmentation

What is it?
In the context of maintaining filesystems, defragmentation is a process that reduces the amount of fragmentation. It does this by physically organizing the contents of the mass storage device used to store files into the smallest number of contiguous regions (fragments). It also attempts to create larger regions of free space using compaction to impede the return of fragmentation. Some defragmention utilities try to keep smaller files within a single directory together, as they are often accessed in sequence.

Causes of Fragmentation
Fragmentation occurs when the file system cannot or will not allocate enough contiguous space to store a complete file as a unit, but instead puts parts of it in gaps between other files (usually those gaps exist because they formerly held a file that the operating system has subsequently deleted or because the file system allocated excess space for the file in the first place). Larger files and greater numbers of files also contribute to fragmentation and consequent performance loss. Defragmentation attempts to alleviate these problems.

How to Defragment?
Most, if not all Windows Operating Systems come with a Disk Defrag ultility which can be found in the accesories, system tools folder.
Other programs such as Defraggler offer you the option of defragmenting the whole drive (which most other utilities offer) or to just defrag a certain file(s).