$Fragmentation 
When a file is stored on a disk, the operating system, DOS, normally tries to save it in one block. But if your disk is very full, there might not be enough room to save the entire file in one block, so DOS splits the file up and stores the separate sections in any available gaps. This spread of bits of files over the disk is called fragmentation. It's not dangerous, but it will slow down your disk access time since the disk head has to move to several different parts of the disk to make up the entire file. There's a special program included with DOS and Windows 95 called Defrag that will go through the entire disk and sort it all out so that all the files are in complete blocks. You should try to run Defrag every month or so to keep your disk drive at peak performance. (See also Defragmentation.)