If your computer seems to be running a little slower than normal, a disk defragmenter is one possible way to speed it back up. A disk defragmenter isn't a miracle cure, but it does have the power to take care of some nagging little problems that sometimes slip through the cracks. It's a minor bit of maintenance that can make a real difference.
When your computer stores files, it doesn't put them all in one place. Regardless of whether the data is a part of the operating system, a program that you just installed, a program that came already installed on the computer, or a file you just saved, the computer puts one piece here and another piece there. To the computer, it doesn't matter where the files are physically located on the hard drive; as long as it knows where they are, it can find them.
A disk defragmenter is a program that helps the computer organize files more efficiently. As files are moved around, programs installed and uninstalled, and information added or deleted, gaps form. The disk defragmenter program helps the computer to reorganize the data so that there are fewer gaps, and so that the files can be accessed more efficiently.
A good way to understand how a disk defragmenter works is to imagine that the data on your hard drive are books, and that the hard drive is a book shelf. Imagine that one shelf is completely filled with paperbacks -- these represent the files that your disk defragmenter is organizing. When three small files are deleted, a paperback is taken out for each one. If these files are spaced across the drive, you have three small spaces where new data can be stored.
Imagine that your next file is very large, the size of a dictionary. There is no way that file will fit into the small space left by a single paperback. Without running a disk defragmenter, the computer will have to put it on another shelf, another area of the hard drive. If you run the disk defragmenter first, it slides all the paperbacks on the shelf over, leaving one large space instead of three small spaces. Now there's room for the dictionary, the large file.
When your data is spread around on your hard drive, the computer has to work harder looking for it. This can slow down every process. Using a disk defragmenter program helps reorganize the data into a better use of space. Whereas before, the computer might have had to look on four shelves for a particular book, by removing the empty space and organizing the data, the disk defragmenter keeps it all packed neatly on a single shelf.