text: Disk Fragmentation:\nWindows stores files on your HD in sectors.  there's a file called a File Allocation Table (FAT) that keeps track of the filenames, size, and occupied disk sectors of every file. When you delete a file, Windows doesn't actually delete the data of the file, it just removes it's entry in the FAT, marking those sectors as available.  when you save a new file to your disk, it will find all available sectors, and then overwrite the existing data.  when the file is small enough to fit into a space of available sectors that are phisically together on your disk, that is a contiguous file.  the read/write (R/W) head on your HD doesn't have to jump all over the disk to hunt down all the sectors of a file.  This is a good thing, because it makes accessing your data faster. But sometimes, chunks of avialable sectors don't always match the file size of the file you're saving, so it has to use available sectors elsewhere making the file fragmented {red areas in defrag} and your R/W head has to jump all over your disk to find all the data.  Bad and slow.  Contiguous files are good, and it's the point of fragmentation.  The other colors in Defrag are White, free space, and Green, un-movable files.  The green files are the special files that must occupy the same sectors, like your FAT and Master Boot Record.  And now you know the how and why of Defrag.