Parts you may need or be looking for.

There are many types of microprocessors from many different companies the two main types are AMD and Intel. Intel is the leader in the market of microprocessors but being the leader also has the higher cost in microprocessors. Also when selecting the microprocessor you have to think about the motherboard since AMD and Intel are not directly interchangeable and you have to make sure the microprocessor will fit the motherboard socket.
Graphics Cards
Graphics cards have come along way since their invention. There are many different types for many different purposes. There are cards for Playing video games as well as business class cards for less taxing processes.
There are different amounts of RAM for different things. Most business class computers will have 4GB of RAM where as a heavy gamer will have more such as 8-16Gb of RAM and all the way up to 32GB.
Hard Drive
There are different types of hard drives, there are internal and external and there are hard drives and solid state drives. The difference between hard drives and solid state drives is that a hard drive has moving parts and a solid state drive does not. Also the solid state drives are quieter as well as faster and are better for use with programs or operating systems that do not often change, but to go with this speed and sound reduction there is a hefty price to pay with a solid state drive and you do not get as much storage as with a standard hard drive.
Power Supply
A power supply is crucial for a computer because without one it will not run at all, so you need to select the correct power supply for your needs. Most people suggest not to go for the bare minimum because it can cause more problems when trying to find something that fits your components.
Computer Cases
There are different types of cases for different types of uses. For people who have heavy processing work or for gamers a bigger case would be more beneficial for heat dissipation, where as for people who use their computer for more simpler things like word processing than a smaller case would work just fine.
Cooling Systems
Cooling systems can range in type and price from a standard fan to a liquid cooling system, and from $30 to over $160. Most people would buy just a fan for about $30 but some people who have heavy processing will go for the higher end liquid cooling systems for a lot of money, but with the higher cost there is also the possibility of leaking and harming the hardware.
When building or upgrading a computer the motherboard is everything since everything in a computer connects to the motherboard in some way. When selecting what motherboard to use you need to understand what you want your computer to be able to such as, gaming, web browsing, or video editing. When selecting a motherboard you need to make sure that the microprocessor that you have selected is compatible with the socket on the motherboard. You also have to think about whether you want to reuse the memory slots and make sure the memory will work with the motherboard. Also make sure that the video card(s) will fit in the slot(s) you have selected on the motherboard.