The hard drive (HD) is where you keep stuff. It is on there permently, even when you turn the computer off. It's the thing that makes that noise when you are loading a program or something.
Your memory is a half way house for the data to stay between the hard drive and your computers brain, the CPU.
Have you ever noticed that when you first turn the computer on, and load up say internet explorer, it takes maybe 4 or 5 seconds to load up. Now if you close IE, then load it up again, it loads up within like a second? Thats because the first time round, your computer is getting the IE program from the HD.. which is one of the slowest pieces of machinery on your computer (due to it's physical limitations).
While it's getting IE from the HD, it also loads IE into the memory. So then the next time you goto open IE, it gets it from the memory, rather than the HD.
At the moment, IE is taking up 25 megs of my memory of my computer. I have 256 megs total. So if I close IE and loaded up something thats really memory intensive, like a game or something..then thats going to take IE's place..so if you load up IE aftr playing a game, it'll have to access it from the HD again.
The reason why you want to defrag your disk is cause of it's physical properties. But uhh... yea Ill tell you why if you ask...cause im going to stop this post before it comes a novel.
What gravity?