Defragment: (from my opinion) it’s an operation for your files to rearrange your files in your HD, making one and other files that have coherency ordered so that’s your hard disk can work efficiently. You can see the flash below to understand better (I got it from other website here)
In Windows XP (yeah, I’m still using it ), you can run the application on: Start -> All Programs -> Accessories -> System Tools -> Disk Defragmenter. Or you can find it on Vista/7 with “Disk Defragmenter” on your Start Search. Thanks for linux-ers, as far as I know (CMIIW) you may not use this program because of the system file. You can google it for ext*.
And again, as far as I know (I’ve read it on a computer magazine), it said that the operation on doing this activity is tougher than formatting your HD. But, some articles said vice versa. So, it’s depend in your need after all. Last: FYI, don’t too often to defragment your HD. It’s better to do it regularly, maybe once every 2 until 4 months, except you do install and uninstall many programs on short time, you must check your HD then.