For many of your questions, Wikipedia is your friend. Some of the answers they give are rather detailed and you may find much confusing there but I will suggest you refer there often.
To cover your questions in the most basic manner I can, here goes
Where to start? Here at Tom's of course! There are 8 basic parts to a computer (getting to be 7 anymore - I'll explain that later)
1. CPU is the Central Processing Unit. This is where most all computations take place. The speed of most current CPU's is measured in GHz (Gigahertz or 1billion cycles per second) and is made up of two parts, the base clock and the frequency multiplier - those can get manipulated for overclocking - some games benefit from overclocking. The CPU produces a lot of heat during operation and so a heatsink is used on it to keep it in operating temperatures. CPU's are socket specific so the socket of the CPU and motherboard must match. Only two brands that matter much for gaming Intel and AMD
2. Motherboard is the platform the CPU and ram and graphics card call home. It is partially responsible for ensuring the proper voltages for components that reside on it and for routing data from one component to another. Good brands are Asus, Gigabyte, MSI, Intel, ASRock
3. RAM Random Access Memory this is the area the computer stores data it is getting ready to use - think of it as the line to use the CPU but on the CPU's terms. Ram is usually described by speed (frequency) in MHz (1million cycles per second) and currently uses a DDR (Double Data Rate) architecture with another set of numbers called the CAS latency. The latency numbers, sometimes just a single number but usually expressed as series such as 9-9-9-27 or 8-9-9-27 represent a delay (lower numbers are better here) in the cycles it takes for the CPU to request data fro the ram until the ram supplies the data. Ram can be finiky and some ram just can't get along with some motherboards and sometimes other ram. Ram is best purchased at one time in a kit - mixing and matching ram is not suggested. Good brands include Kingston, Corsair, GSkill, Samsung, Crucial, Patriot, Geil
4. PSU Power Supply Unit This is probably the most overlooked part of a computer build. It's importance to the proper operation of the computer is so great and yet for most, little time is spent into researching what makes a good PSU. There are some excellent PSU's on the market, there are also potential disasters posing as PSU's. Power supplies have a wattage rating that should not be exceded, in fact most recommendations are that the PSU be double what the system will draw. Good brands include Antec, Corsair, SeaSonic, FSP, PC Power and Cooling, XFX, ThermalTake but it is very important to check real reviews for any PSU you are interested in because there are some really crappy ones out there
5. GPU Graphics Processing Unit is responsible for painting picures for your monitor. The GPU can be located in the CPU, on the motherboard itself or even stand alone (dedicated). A dedicated Graphics Card is important when gaming. The GPU can, in some cases, take some of the processing load off of the CPU and often improves gaming. The general rule for graphics cards for gaming is to purchase the best one you can afford. There are two basic dedicated graphics players, NVidia and AMD (same AMD as CPU maker but not limited to AMD motherboards - they play happily on Intel boards also). Some graphics cards canbe attached to others to make a better graphics solution but the maximum amount of graphics cards you would want to run together is 4 (but only for extreme uses) - there are restrictions on how they need to be to work together properly
There are many manufacturers of graphics cards but they all use either Nvidia or AMD GPU's
6. Case or Chassis is what holds all of the components in one area - there are some cheap crappy cases out there, there are some great inexpensive and mid priced cases and there are cases you can't consider until you win a lottery (or just about). As far as I am concerned, case choice comes down to personal preference. Some things to consider about cases are connections (USB 3.0, eSATA etc.), size and airflow (remember, the CPU and some other components get hot) Too many choices here to recommend specific brands
7. Hard Disk Drive or Solid State Drive. These are your storage areas for data. Virtually everything the computer knows is stored there. All data and programs get loaded to there and launch from there. SSD's are much faster than HDD's. Western Digital and Seagate are popular HDD choices, Samsung and Intel are popular SSD choices
8. Optical Drive A DVD or CD or Blu-Ray drive used to be required for a build, not so much anymore, operating systems can be loaded through alternative means and much content that used to be the realm of optical drives is now available to stream on-line. Most builders simply buy the cheapest available
Last thing is size - case and motherboard, the motherboard needs to be able to fit in the case. Motherboards come in sizes from (smallest) Mini-ITX, Flex-ATX, Micro ATX, ATX (normal size), E-ATX, EE-ATX. Mini-ITX boards can only support one graphics card (if that). Some smaller cases require smaller power supplies and some of the smallest cases (I won't suggest for a gamer) have external power supplies similar to a laptop's.
I think after checking out Wiki for some further reference, you may find you have specific questions - best to deal with each one in it's own thread later IMO. I can also supply some specific reading if wanted
These videos although quite dated now are great IMO for learning some of the basics
1
https://www.youtube.com/watch?v=lPIXAtNGGCw
2
https://www.youtube.com/watch?v=d_56kyib-Ls
3
https://www.youtube.com/watch?v=RxaVBsXEiok
That is the most basic summary I can do here. Hope it helps some
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