Parts of a Computer
Case
Your case is the physical structure and protection for the internal parts of the computer. The size of your case determines the biggest size motherboard and if it's possible to fix other components. Often Fans are included with the case but may have mounts for extra fans.
The sizes from biggest to smallest:
Full tower
Mid Tower
Micro-ATX Tower
Mini-ITX Tower
Motherboard
The motherboard is the component that physically connects and ensures that parts of the computer can talk and interact with each other. They come in several form-factors. The Motherboard determines which CPU. This is because the motherboard has a specific socket that only fits specific CPUs. Ram is also another component. Only one DDR (Double Data Rate) Generation is suitable for a motherboard. Note the back panel display ports are only usable without a dedicated GPU else the GPU ports must be used.
CPU Central Proccessing Unit
The CPU is the part that does the calculations of a computer. It controls the interpretation and execution of instructions and calculations. The CPU can sometimes also contain a GPU. This GPU is called an integrated graphics card and will be weaker than a dedicated Graphics card and has less VRAM (video RAM). Note if your CPU doesn't have an integrated graphics card then a dedicated one must be installed for the PC to work as without it the computer can't display anything on your monitor. It is measured by its clock speed (actions that it can calculate per second), FLOPs (Floating point operations per second) and Cores (How many separate tasks can be done at once). Sometimes the CPU comes with a stock cooler.
RAM (Random Access Memory)
Ram is your computer's short-term memory. Often called computer memory. It stores data that your computer needs to access run apps and open files. It is volatile which means once the computer is powered off it will reset. It is faster and smaller capacity than non-volatile storage such as SSD or HDD. DDR5 is the current generation and its capacity is measured in GB such as 16GB or 32 GB and speed is measured in MegaHertz or GigaHertz. If your computer runs out of RAM it may try to use Storage as an increase of compacity but this will not perform as well as RAM.
The best visual of RAM being used is when a game loads in a level and you see that progress bar.
GPU (Graphics processing unit)
The GPU is how a computer displays and does graphics-related calculations (Video editing, rendering etc.). It is essentially a combination of CPU and RAM designed for only these types of calculations. The VRAM (video ram) size is how much working memory the GPU can calculate while bandwidth is the read and write speeds. Similar to a CPU both Clock Speed and Flops are also used to measure.
PSU
This acts as the source of power and transforms power from the wall outlet into usable voltages that work for specific components. This is one of the most important parts of a computer as an underpowered, cheap or damaged PSU can damage every other part of a computer. Typically measure in Watts. The PSU supplies power directly to the motherboard, dedicated GPUs and non-NVMe Storage.
Storage
Storage is your computer's long-term memory. Traditional HDD (Hard Disk Drive) and SSD (Solid State Drive) use SATA (Serial AT Attachment)cables to connect to the motherboard. While newer NVMe SSD is inserted like Ram directly into the motherboard. Storage is measured in Giga/Terabytes, read and write speed in MB/secs as well as write cycle (number of times expected that the disk can be erased with new information written over) for durability.
Choosing a PC Parts
First, you need to devise what type of usage is needed. The 2 most important for your performance are the CPU and GPU if they are imbalanced it can cause a bottleneck due to the weaker part. The amount of ram is decided by usage again, a good minimum is 16Gbs. Storage is up to how much storage you need. Once those a selected choose a compatible Motherboard, PSU and Case.