The typical PC is made up of just a few parts. Inside the case, there is a usually green or red motherboard. On this motherboard there are many expansion ports, such as SATA, power input from the power supply, and the crown jewel: the CPU. This is the main chip that calculates everything that you need to do billions of times a second. The CPU is usually underneath a large heatspreader and fan because of the usually intense heat output. Also plugged into the motherboard is usually a graphics card in higher end systems. This is now done in modern days by a PCI x16 slot. This port connects anything plugged into it directly into the CPU via PCI lanes. In older days it was done via the southbridge, an older form of architecture. The final thing that is plugged into the PC is a power supply. This metal box contains a transformer to step down the voltage to levels that the computer can use. This is typically 5 volts for USB, and 1.3ish volts for the CPU.