CPU = the brain of your computer. Handles all the calculations and tasks and it's the "thinking" of everything your computer does. The faster and more powerful this is, the more of a difference you'll notice in your day-to-day computing. With a really fast CPU, your computer will not delay or stutter almost whatsoever.
RAM = random access memory. This determines how many random tasks your computer can do at once. The more RAM, the more programs you can have open, simplistically speaking.
HDD = hard drive. It's where you keep your files.
GPU = graphics card. This determines how powerfully your computer can produce graphics. The better the card, the higher up the graphics settings you can crank on video games, the better the resolution (like 1080p or 720p or whatever) your computer can support, and the more monitors your computer can support (among other things). Anything graphics related is determined by this.
Wireless cards = these let your computer connect to the internet wirelessly. If you have a wireless internet connection that your computer connects to, it does so through the wireless card. N/G/B/A. N is the fastest new standard, although still needs some improvement before it's as reliable as the old G. B is slower, A even more slower (I think?). If you have a wireless card that supports N, and a wireless router that your card connects to that also supports N, you'll have the fastest wireless connection possible. Other factors include 5GHZ or 2.4GHZ (or whatever) channels. If your wireless card/wireless router run on 5GHZ, it's better because it's a more uncommon frequency so there's less interference with the signal. If you run on 2.4GHZ, things like microwaves and phones use that frequency as well, so there's more interference. You can get USB wireless adapters now, though, so you don't have to use a card.
Motherboard = It's a flat board that everything mentioned above "lives" on. Think of the motherboard as the neighborhood and the computer pieces as houses living in it. Everything plugs into the motherboard and is connected to each other through it. It looks like a flat board.
USB = Universal serial bus. Used by the famous "thumb drives" or "flash drives". USB is really the type of connection these thumb drives/flash drives use. If you have a USB thing, that means it can plug into your computer through its USB port. USB 2.0 is the standard, but the much faster USB 3.0 has come out and new computers and gadgets are using it now to transfer data faster than before (the reason why it takes so long for your iPod to download/transfer new music to itself is because USB 2.0 is so slow :<).
PSU = Power supply unit. These are the blood vessels of your computer. It plugs into your motherboard and supplies all the electricity to it so that everything can work. It's where you plug in the electrical cord from the computer to the outlet.
Sound card = This is what your computer uses to produce sound, and it's usually found in one of the PCI ports in the back of your computer. If you don't have a sound card, your motherboard will usually handle it by itself (the default ports on the back of your computer). Depending on the sound card you get, you can have higher quality sound than anything you'll find anywhere else, including surround sound and digital signals, and you can even record music. If not, you'll have to deal with the default through your motherboard.
Cooling = The system your computer uses to cool itself. If its components get to hot, they will stop working, and since they do so much, they get hot very easily. Thus, your computer has a system of fans that cools itself to keep itself running. If you're fancy, you can buy a liquid-cooling system that is much quieter and cools your computer with, yep, liquid.
You can buy all these parts separately and assemble them on your own to have a computer. You'd need a few extra things (like a case, of course, and a DVD drive of sorts, and a copy of Windows or OSX, duh), but in a crude sense, you'd have what you need to make a functioning computer.
Other things you might want to know:
*On-board graphics = this basically means that you don't have a GPU. Instead, your CPU handles the graphics itself straight from the motherboard. Thus, it doesn't have its own GPU to handle graphics intensive applications. This is very cheap, low-power, but don't EVER rely on this for anything like video games or streaming YouTube without skipping. It's basically what most laptops have so that they remain inexpensive. But this is why you can't really play video games on laptops; because they usually have on-board graphics and not a real GPU that would let them play most modern video games well.
*SATA/E-SATA, etc. = These are just cables and ports you will want if you want a fast connection from your motherboard to something else. For example, if you want a DVD drive that rips discs quickly, you're going to want to hook it up on an E-SATA port on your motherboard.
*Thermal paste/heatsinks = If you ever buy a CPU, it will come with a heatsink that basically protects it from the rest of the computer and allows its heat to diffuse somewhere rather than just hang around the CPU, I'd assume. Before you attach the heatsink, you'll have to put on a good "thermal paste" right on the CPU (for protection against heat). It's like spreading toothpaste on a tooth.
*mounting/static bracelet = Don't ever open your computer and work on it without mounting yourself with a static bracelet or touching something metal like a refrigerator before getting started. You need to drain the static electricity built up inside your body before you can touch the components. If you touch your CPU or motherboard with too much built up static electricity, and it shocks the system, it can break your CPU or motherboard forever and you'll have to replace it. Considering these can cost from $100-1000, DON'T RISK IT.
*AMD/Intel = These are the two major CPU manufacturing companies. AMD CPUs tend to be cheaper, but Intel CPUs tend to be better, although more expensive. If you want an Intel CPU, you're going to have to pick out a motherboard that supports Intel CPUs. If you want an AMD CPU, you're going to want to pick out a motherboard that supports AMD CPUs. Note: Intel motherboards work fine with either ATI or Nvidia cards. AMD motherboards that work with AMD CPUs usually only work with ATI graphics cards, and it is essentially important to research the motherboard you want if you want a feature like Crossfire or SLI.
*ATI/Nvidia = These are the two major GPU manufacturing companies. Every year one tends to slightly outdo the other in terms of power/performance/cost, so don't worry about it. Just go for which one you like better and better fits your budget and system. It is possible to buy multiple graphics cards and hook them all up together. Two or three heads are better than one, right? For Nvidia cards, this is called SLI. For ATI cards, this is called Crossfire. Your motherboard has to support this feature in order for you to use it, so note it when looking for motherboards if you're interested in using multiple graphics cards at once. For some graphics cards, it is also possible to hook up your cooling system directly to the card for maximum temperature regulation and cooling control.
*Dual-Channel/Triple-Channel RAM = Just know that triple channel is faster than dual channel. So if you want RAM and you want the fastest, triple channel is the way to go. DDR3 > DDR2.
*SSD = Solid state drive. This is a different from HDD hard drives (hard disk drives) in the sense that there is no disk. Conventional hard drives spin like a disk, eventually wear, tear, and fail, and their speed is dependent on how fast they spin (3600RPM, 7200RPM). Solid state drives are all flash memory. Thus, they retrieve information INSTANTLY (or at least nearly instantly) and they last much much much longer than HDD since they don't have any moving parts. You can boot up a computer whose copy of Windows is installed on an SSD in literally a matter of a few seconds. You want this if you want programs that need to instantly retrieve important information, like Adobe Photoshop, or whatever, if you're a media professional.
*Dual-Core/Quad-Core/i7/etc.= This is how many "cores" a CPU has. Once CPUs went dual-core, it's like you had two CPUs in one. So basically, your different cores could handle different tasks at once. Like how it's easier to clean up a room when you delegate tasks to people who suit them the best. Same concept applies later when we got to quad-core. And i7 CPUs (and AMD's alternative Phenom X6 CPUs or whatever) run 7 cores, although only the high-end i7 CPUs "truly" run on seven cores. The lower-end i7s are four-core CPUs that run with "hyperthreading" plus other shit I don't know about that makes it simulate having seven cores. The more cores and the higher your clockspeed (which is the GHZ) the faster your CPU, the smarter your computer, the more stuff you computer can do.
*512MB/1GB/etc. GPUs = This is essentially how much memory your GPU has. The higher it is, basically the better it is (in a layman's sense). a 1GB GPU has more memory than a 512MB GPU (and if you didn't know, 1GB = 1000MB, so it's like 1000MB > 512MB). There are other aspects that determine a GPU's power/speed, but this is one of the simpler, more important concepts you should understand.
*PCI/PCI-E/RAID = these are the terms for the expansion ports on your motherboard. Think of it like upgrading your main character in a game, or adding the expansion pack to the N64. With more PCI ports, you can buy more things like graphics cards, sound cards, wireless cards, video capture cards, what-have-you to plug into your motherboard from the back of your computer. PCI-E is the fastest, and the rarest. PCI is the second-fastest. RAID is the slowest, you'll hardly find it in computers anymore, or anything that supports it for that matter.
I have been browsing Alienware, CyberPower, and IBuyPower.
Digital Storm is by far the best place you can order a custom computer. It's renowned by the BBB and customers and they just really do a good job, apparently. But they're expensive...
..So I used IBuyPower. Had my PC for 6 months now and it's fantastic, runs beautifully and as advertised. <3
