At its core, a computer system is nothing more than a combination of computer parts working together. The most essential parts are the CPU, motherboard, memory (RAM), power supply, and hard drive (or SSD). These are typically housed in a computer case unless you’re building a test bench where you’ll need to swap out components on a constant basis. Most computer system builders build their own computers for gaming so if that’s your goal, then you’ll definitely want to look into graphics cards.