Deframenting a disk is the process of moving little pieces of information from scattered places on the disk into a compact sections, so that more information can fit.
Contrived example: Say you have 15K available on your disk:
+++++++++++++++
You create a file that’s 5K
XXXXX++++++++++
You create another file that’s 3K:
XXXXXYYY+++++++
And a third that’s 4K:
XXXXXYYYZZZZ+++
Now you have 3K left. You want to create a file that’s 5K. Obviously you don’t have enough space. So you delete the 3K file:
XXXXX+++ZZZZ+++
But you still don’t have enough space. Your computer can’t find any place for that new 5K file to go. So you defragment your drive:
XXXXXZZZZ++++++
Now you can create your 5K file:
XXXXXZZZZAAAAA+
Think of it as similar to compressing your trash basket. You know you can fit that empty box of rice in the trash, but first you have to press all the garbage down to fill in the individual bits of free space, before the box of rice will fit on top.
This also has the benefit that your computer can access existing data more quickly, because all the data is compacted to one area of the disk, rather than spread out over the whole thing.
In Windows XP, you can find the Disk Defragmentor tool by going to Start -> AllPrograms -> Accessories -> System Tools -> Disk Defragmentor. If you’re using another operating system, try to find the nearest equivalent location.
Hope that helps!