When your computer goes to write a file to the hard drive, it looks for the first available blank spot on the disk to write it in. Ideally, the computer writes out the file in one big block of information. However, if that open section of the drive is not large enough to write the whole file, the computer will fill the empty space and go find the next empty space and continue writing the information. The computer will continue this process of looking for open space and filling it in until the entire file has been writtten, all the while keeping track of all the different places it has put these file fragments. Disc defragmenting is the process of putting the fragments of each file back together in one coherent block of information. Defragmenting your hard drive speeds up your computer's ability to access an individual file because all the information of a given file is together and not spread out over the hard drive. These empty spaces that lead to disc fragmentation are caused by the deleting of files. These files can be anything: data files, program files, or operating system or browser temporary files. A good rule of thumb for defragmenting a hard drive is once every 3 to 6 months. The more heavily you work your computer, the more frequently you should defragment your hard drive. Some computer file systems don't need to be defragmented. Most Windows/DOS files systems need to be defragmented, while Unix/Linux files systems do not.

I would just like to add that defragmenting every month (or maybe even more) might be a little more appropriate if you: Install and uninstall a lot of programs on a regular basis. Surf the web a lot and delete your temp files on a regular basis. And I only offer the suggestion to defragment more often in these situations because the more frgamented your drive is, the more it impacts on performance and the longer it takes to finish the defragmentation. If you like your defrag to take just a few minutes - do it every week. (I have a weekly routine - delete temp files / virus scan / scan disk / defrag / scan disk)

The other posters have covered the basics of disk fragmentation fairly well. Note, however, that as more data is stored on a disk, the more likely it is that file fragmentation will occur. I do not recommend ever filling a FAT- or NTFS-format hard drive beyond 80% capacity. Heavy fragmentation will occur if the disk is too full. The type of software that you use may also promote fragmentation. There is one data analysis package that I use which causes very heavy fragmentation, resulting in a sharp drop in performance. When I am working with it extensively, I often defragment my working disk every day. If you are managing and editing very large files, particularly video files for DVD authoring, you will also find that the disk fragments badly. You have to use a large drive (e.g., 200 GB) and defragment the drive every few days.