You have a kitchen (Computer Internals). This kitchen is placed inside of a building (Computer case), and this building is fed gas and electricity (Power Supply or PSU.) Everything within the kitchen is placed on the kitchen floor (Motherboard). There is a pantry (Hard Drive) that holds all the raw ingredients that you're going to work with, and the counter top (RAM) is where you prepare them. The stove top (video/graphics card) is where meals are finally prepared and dressed up . The chef (CPU) works with all things on the kitchen floor. Pretty simple so far, huh?
Getting a bigger pantry (Hard Drive) doesn't mean you'll be able to prepare anything in there any faster. A bigger counter top (RAM) won't make your chef (CPU) work any faster, but you can have more things out on the counter (Temporary Memory) for your chef to work with. If you have a great stove (video/graphics card), it'll make whatever you put on it look as good as it can, but if the pantry (Hard Drive) doesn't have anything of high quality, the stove (video/graphics card) will never be put to good use. Improving your chef (CPU) will improve your ability to select choice ingredients (read and execute programs), but certain chefs will only work on some kitchen floors (Motherboards.) The kitchen floor (Motherboard) determines what your kitchen can support. If the kitchen floor (Motherboard) isn't stable enough, or if the other sections aren't compatible, then everything will fall apart.
So the computer illiterate comes to understand that the most important component of their computer is the motherboard or mobo for short. The motherboard determines what parts are going to be compatible in your system, namely the CPU. The CPU or Central Processing Unit fits into the motherboard and works with all the programs and applications that you're going to be using. The Video Card determines the visual quality of said programs, and the Hard Drive will determine what and how much you can hold. Finally, the RAM or Random Access Memory dictates your computers capacity for multi-tasking. The more RAM you have, the more programs you can have open at the same time.
I hope that helps the computer illiterate out there understand what's going on with their computer. While this analogy doesn't cover programs or operating systems, getting one to understand the underlying hardware is a good place to start learning about these amazing machines. Hope this helps!