A file system (often also written as filesystem) is a method of storing and organizing computer files and the data they contain to make it easy to find and access them. File systems may use a data storage device such as a hard disk or CD-ROM.

File system fragmentation, sometimes called file system aging, is the inability of a file system to lay out related data sequentially (contiguously), an inherent phenomenon in storage-backed file systems that allow in-place modification of their contents. It is a special case of data fragmentation. File system fragmentation increases disk head movement or seeks, which are known to hinder throughput. Fragmentation occurs when the operating 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 operating 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. The correction to existing fragmentation is to reorganize files and free space back into contiguous areas, a process called defragmentation.

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 (defragmentation software) also try to keep smaller files within a single directory together, as they are often accessed in sequence.

Before we learn the defragmentation steps make sure that:

• copy or backup all your work files, photos, email
• close all your currently open programs – media players, virus scanners and other programs (also those with icons in the system tray)
• assure your computer has a constant source of power

To defragment your hard drive you should:

1. Open the defragmentation program: Start -> All Programs -> Accessories ->System Tools -> Disk Defragmenter

2. Let Windows to determine if you need to defrag your drive

3. After analyzing disk, Windows will tell you what to do next. If your system files performance can be improved it’s recommended to defragment your disk.

4. Start defragment your disk by clicking on “Defragment now…” button. Select the disk(s) you want to defragment and click OK.

5. Now be patient and let your PC doing his job. It can take from a few minutes to a few hours.

When the job is completed, you can close the program window. You may experience some errors. If this happens, save the report file for further usage (maintenance or repair).

Info via Wikipedia.