I guess I'll explain all the parts to you:
Case, Can affect your computer's performance based on how well it disperses heat.
Power Suppy, judged based on wattage, this takes the electricity from your outlet and distributes it to all of your parts.
Motherboard, the center of everything, houses the Micro Processor (CPU) and has extremely basic versions (From now on called "onboard") of all the other "unnecessary" parts.
Hard Drive, Stores all files on your computer, including your operating system (OS). Judged based on capacity which is represented in bytes, 1024 bytes is a kilabyte, 1024 kilabytes is a megabyte, 1024 megabytes is a gigabyte, and 1024 gigabytes is a terabyte. Multiple hard-drives can be installed, one being the master, or higher up than the others, (this one houses the OS) and the rest being slaves.

CD Drive, reads your discs that you put in, judged based upon quality to which it can scan(bluray vs dvd).
multiples can be installed.
CPU, Placed in the middle of the motherboard under a large, individual fan, this processes all your information, judged based on hertz. Ex: an average computer would be maybe.... 2 gigahertz?(I am not extremely educated on cpu, you may want a second opinion on what its judged upon.)
Graphics Cards, Houses upon top of the motherboard. There are two major brands of graphics cards, nvidia and ati, I personally prefer nvidia. I know how nvidia cards are judged, but not ati. Nvidia cards speed is based upon increments of 100s, and is currently in generation X200s(10200s). Graphics cards process textures and get them to your screen, they share a joint burden with RAM (Random Access Memory) and you need ample of both to run speedily. You can have more than one graphics card with technology offered by the company who makes them.
RAM, or Random Access Memory is housed on top of the motherboard. This is what jointly shares the burden with your graphics card of getting your picture to your screen. While not sure of the types of RAM, I do know that it is based upon bytes also. 2 Gigs is a common recommended requirement of modern games. You can have multiple sticks of RAM, but they must be of the same type.

Now these are all the internal parts I find important, and the only parts on my computer, though you can get much more. Such as sound-cards, which enable high performance sound, network cards, that increase network speed, and much more.
I do not know everything about computers and if you can improve upon the information here, please do so. I was looking for a picture of a full computer from the side, but I could not find one.