I am pretty sure that's the guide I used.
Anyways, so here are the main parts:
Case: Holds everything together.
Power Supply: Has a bunch of connectors that connect to everything that needs power in your computer (cd/dvd drives, hard drives, Motherboard, graphics card, etc), this is the piece that plugs into the wall of your house.
Motherboard: This is the piece that looks the most complicated. It has tons of little everythings. Everything on the computer hooks to this in on way or another.
Processor: This is a little square chip. It plugs into the motherboard. It's hard to describe basically what it does, but it's a very important piece. (when people say q6600, or q9650 they are talking about the processor)
RAM: Random access memory, it's basically there to more stuff along, and to hold a little bit of data while you're switching between programs and stuff. The more you have, the more stuff you can have open, etc. The faster it is, the faster you can switch between stuff, etc. (of course this depends on loading off the Hard drive, and the processor speed and stuff too)
Graphics card: What you need to play games. Some new ones just came out. (nvidia 8800gt, and the new gts's, great value as far as I can tell). Different ones need more or less power depending. Most new decent power supplies can support all new graphics cards.
I am going to assume you know what a hard drive is, and a cd/dvd drive.