The parts that make up the back of a computer are as follows:
CPU (or "processor")- This often described as the "brain" of the computer. The speed of the processor, measured in GHz, determines how many things the computer can do at the same time, and how quickly. Processor speed is of particular importance for multimedia programs such as video editing.
Motherboard- The motherboard is a circuit board that all components are attached to.
RAM: "Random Access Memory", is your computers short term memory. It is used to run different applications at the same time. The more RAM, the quicker the programs will run.
Graphics card: A processor responsible for the visual images you see on the screen, eg. games, videos.
Hard drive: A spinning disk which provides permanent storage to all of your data, including the operating system (Windows, iOS etc) and all of your files. The size of a HD is measured in gigabytes or terabytes.
Power supply: this one is fairly self explanatory. Supplies electricity to run all of the hardware and components.