ok. Here is the basics. You will have to learn more about each, and Ill attach a wiki link to each part for a little more info.
I would also look at your local school to see if they offer computer engineering (only 7 schools in my state offer them, 8 this year (first year for dalton Mcmichael). That would give you a good bit of knowledge on the subject.
Case:
This is the most visible part of a computer, much like the body on a car. Just like cars, there are different sizes.
ATX Full Tower - The biggest. Like a lorry, they have all the room you could ever need. They generally have 6+ 3.5 drive bays, and room for full loop water cooling. Examples: HAF, Antec 900.
ATX Midtower - intermediate. Like a pickup truck. Room for the whole family, and some equipment, but not so large as to be unwieldy. These are the most common cases used in custom built computers. Examples: HAF912, Antec 300.
mATX- these are like a midsized car. Room for all the essential components, but not much else. Good for HTPC, or general office work, but not much else. If using a dedicated video card, you will need a midrange card. You will have issues fitting anything large in the case. GT520, HD6570 or such is about the top end for these cases. Limited room for Hard Drives and optical media drives. These cases require a mATX or ITX board.
ITX: Room for a ITX motherboard, 1 or 2 3.5 HDDs and a slim optical media drive. Used to house a weak processor (or sometimes a laptop processor) to control a television or other large screen streaming device.
http://en.wikipedia.org/wiki/Computer_case
Motherboard:
This is like a transmission. they come in two verities, Locked BIOS, that comes installed in nearly every name brand computer, that just allows small changes such as boot order, time zones, HDD password or the sort. Much like a A4LD (google it, lol) transmission, these are pert near useless. The Unlocked BIOS versions are what comes in custom built computers. They allow for maximum customization to the components through the BIOS. These are used for Overclocking and optimization. Comparing them back to the transmissions, this would be a TH400, C6, T19, NP435, NV4500 or such. Tough, but dependable and useful.
Motherboards have many parts to look at when considering buying one.
-CPU Socket. (LGA or PGA)
-GPU sockets (PCI, PCIe, AGP)
-Expansion slots
-Chipset (north bridge, and south bridge for AMD)
-Size (ITX, mATX, ATX)
http://en.wikipedia.org/wiki/Motherboard
Central Processing Unit (CPU or Processor)
This is like the engine in a car. Generally, the higher the number (955 vs 1090 or 2300 vs 2500), the better the performance. Much like the engine though, you dont always need to get the biggest. For example, would you really need a 572 in a Vega? Just like that, you dont need a Hexa core beast to run a DVD viewing app.
these are 2 big names in computer processors Advanced Micro Devices (AMD) and Intel. Put this like Ford and Chevy (holden and ford, or whatever the two big names in your country happens to be).
AMD would be Chevy. Keeping the ability to retro fit down the line. They are currently far behind in the performance market (AMD, not chevy). Their 955, 965, 970, 980, 1090T, 1100T and so on black edition processors are like a good old small block, be it a 265, 350, 400, or 305. Built to last, and on sockets that are backwards compatible. They can be overclocked via the multiplier, like adding a bigger carb and a turbocharger.
Intel would be Ford. Good performers, but not concerned with keeping the ability to reuse old parts. the current line of 2500k, 2600k and 2700k is like the 5,4 and 6,8 modular fords. They have amazing base power, and can easily be OCd via multiplier to amazing speeds (upwards of 5GHz on air).
http://en.wikipedia.org/wiki/Central_Processing_Unit
RAM:
this is like the Timing chain (or belt, or gears) in an engine. The RAM stores data that the computer is currently using. This is essential to the operation of the computer. Without RAM, the computer ceases to function at all.
Ram comes in several flavors. SDRAM, DDR, DDR2, DDR3 and so on. DDR3 is the current standard and 1600 MHz is a pretty standard speed these days. They key in RAM is to look for good timings CAS9 or below and the right seed, 1600 or 1866 MHz on average.
http://en.wikipedia.org/wiki/RAM
GPU:
This is like the dash gauges in a car. its how you get info on what the other parts are doing, be it processing DATA, running a game, processing MP4 video data, or what not. Its the single most important piece of hardware you can have. without a video card, you can not use your computer.
Not all Video cards are the same. Some are general purpose, some are medium duty and some are high performance works of art.
AMD/ATI is the value leader. Their lines have generally, since the HD3k line, run like this.
-HD*4** and below are basic GPUs. Like the factory setup in most cars, Fuel, Speed, Oil pressure, Voltage, but not much else.
-HD*5** and HD*6** are medium duty. They will handle some games at low to medium settings. Its generally the midgrade setup. All the above plus a rev counter.
-HD*7** and up are the gaming cards. They will handle most everything. these cards are like having a pyrometer, vacuum pressure, oil temp. transmission temp and more added in.
Nvidia is the performance leader.
GT is the low end.
GTS is the mid range
GTX is the top end.
http://en.wikipedia.org/wiki/GPU
PSU:
this is like your battery, Alternator/generator, starter and all attached parts. Just like you need to match your battery to your car, you need to match your PSU to your components.
http://en.wikipedia.org/wiki/Power_supply_unit_(computer)
there are also other parts, but these are the most common problem areas as far as hardware goes.
