When files are created, deleted, or modified it's almost a certainty they will become fragmented. Fragmented simply means the file is not stored in one place in its entirety, or what computer folks like to call a contiguous location. Different parts of the file are scattered across the hard disk in noncontiguous pieces. The more fragmented files there are on a drive, the more performance and reliability suffer as the drive heads have to search for all the pieces in different locations. The Disk Defragmenter is designed to reorganize noncontiguous files into contiguous files and optimize their placement on the hard drive for increased reliability and performance.
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). As advances in technology bring larger disk drives, the performance loss due to fragmentation squares with each doubling of the size of the drive. Larger files and greater numbers of files also contribute to fragmentation and consequent performance loss.
The Analyze option will display a summary of the current fragmentation status (the percentage of fragmented files to total files). If you want to defragment the disk without more information, just chose the Defragment option.
It is recommended that you run Disk Defragmenter every 30 days.