There are many different things you chose when building a custom computer. While I can't list ever hardware possibility known to man, I will try to cover the main ones.

For a desktop, there are many different hardware options to choose from.
Motherboard
CPU
Cooling system
Memory
Graphics cards
Power supply
Hard drive

The list goes on and on, but I will try to cover these basics.

-----For the motherboard, there are many different form factor options. From ATX, to BTX, to AT, Micro AT.. Again, I could go on forever.
Here are a few examples:
Powerful: A WTX, for example, is a large motherboard used for high end computers and servers. Using something like this would be far too powerful and costly for your home computer.
Ease of use and less power: ATX/BTX: ATX motherboard form factors are extremely popular, and are one of the industry standards for motherboard form factors. Using an ATX or ATX variant is recommended for a cheap, good computer. BTX form factors are larger with better cooling than ATX, but are not as popular. They need their own kind of slot, and are larger. Either way, they have better cooling, more sufficient spacing despite its larger size, and can handle a bit more than a standard ATX form factor motherboard.
Smaller (laptops for example): Micro ATX, Baby AT, etc. are used for laptops. Use of these in a desktop are highly un-recommended and hardly useful. In a laptop, they can be powerful small motherboards.

Make sure you look for something that will balance power, sufficiency, space, and power needed. You don't want to be running super motherboards in a simple desktop computer.

-----CPU:
The processor of a computer can vary greatly, but there are two main manufacturers I would use. AMD, and Intel. Most of the time, you want a high end processor to fit your needs. Again, there are different kinds of processors, some for laptops some for desktops. I am more of an AMD fan, and use AMD Phenom 9150e Quad-Core. Processors such as Intel Pentium 4s would be a good choice as well. Which processor you get can determine if your computer can handle 32X bit operating systems and programs, or 64X bit. Intel quad core processors would handle 64X bit, while Pentium 2s will only handle a 32X (I believe, just an example).
Make sure, again, you find what you need. Don't overkill the CPU, especially if you do not have proper cooling.

-----Cooling systems:
There are many different cooling systems and methods to use for your computer. Thermal paste is good to keep heat from getting between parts when available. Keep that in mind.

-Heat sink cooling: A heat sink runs liquid through metal pipes to help draw heat from the CPU. It is not uncommon to see a heat sink with a fan as well.
-Liquid cooling: Liquid cooling systems run coolant through tubes around the CPU and other computer parts to keep them cool. This is a bit extreme.
-Freon cooling: If you need a refrigerator cooling system for your computer, then you know its ridiculously powerful. This is what your air conditioning and fridge uses.
-Air cooling: Uses fans to blow air through your computer. This is recommended for most smaller, less powerful systems such as laptops and regular desktops.
-Liquid nitrogen cooling: You should know what liquid nitrogen is. It runs liquid nitrogen around the computer to keep it cold. If you need this then I envy your computer. This would be best used for massive servers.

-----Memory:
Ahh yes, memory. Your computer will only be able to handle so much memory before using more is a waste of time, as it will not become efficient. There are many memory makers, so you should have a good look around to see who best fits your needs. Remember, you can overdo memory to what you might need, but only to a certain amount. I can put 8 GB of memory on my computer in 2 GB intervals. My computer will only handle 7.75 GB, but at least it wouldn't be underpowered. Adding anything more would not have any effect on my system, but my system will use the memory to the best it can process.

-----Graphics cards:
For a graphics card you will have to really dig deep in to what you need. If you are a video game designer, graphics designer, or call for large amounts of media, you want a great graphics card. They come at a price however. Some graphics cards can be integrated in the CPU or motherboard, but from what I have seen more have an expansion slot to connect a graphics card to. If you are just the typical teenaged girl who types random junk to her friends and posts on her blog, a low end graphics card would be for you.

Typically, a graphics card should not run you anything over $150. Chances are you won’t need a 4 GB GeForce GTX. Not even heavy gamers will need that much. Remember, the more powerful graphics cards will need help cooling, meaning better cooling systems, meaning even more money out of your pocket.

-----Power supply:
You will need a power supply. Once you can figure out how much power you need, go about 50% over. It won’t damage your computer since you aren't overclocking, don't worry. This is because power supplies lose about 10% of their efficiency per year. I don't know what else to tell you, there are many power supplies out there but I'm not an expert on them.

-----Hard drive:
For your hard drive, there is raid 0, raid 1, and raid 5. Depending on your need to back up memory and information, you will want the better raid level. Raid 5 I believe requires a minimum of 3 hard drives that mirror to each other, so when one fails the other two keep files backed up. If a drive fails, you can swap it and all memory will be intact. This would be ideal for servers. Raid 0 spreads data evenly across two disks, and is more to increase your overall performance. Raid 1 is a mirror of two disks. That's about all I remember about hard drives from taking my A+, and I believe those are the 3 main raid levels.


That's just about all I have for you, I hope you enjoyed this basic article and have more of an idea for building your own custom computer.