A computer has many parts and each part has its own job. Lets pretend that a computer is a person.
CPU: The CPU is like the brain. It does all the thinking and makes everything work. The CPU controls all the other parts in the computer. Just like your brain controls your body.
Motherboard: The motherboard is like the skin and skeleton of the computer. It holds every part together and makes sure each part can work with one another.
RAM: RAM is like the hands of the human body. If a person has more hands, they can do more things at once and multitask more efficiently. If someone has a job that requires two hands but they only have one hand, it will slow them down. But if a job requires one hand and the person has four hands, then it is pointless to have a lot of hands. Every program on your computer takes up RAM. More RAM = more programs can run at the same time more efficiently. RAM is measured in GB, and it is usually from 1GB to 8GB.
Power Supply: The power supply is like the heart. Just like the heart has to pump blood throughout the body, the power supply has to make sure there is enough power for all the parts in the computer or it will not work. Power supplies are measured in watts and are usually anywhere between 300W all the way up to 800W+.
Hard Drive: A hard drive is like a huge backpack that is being worn at all times. It is where everything is stored. Pictures, movies, documents, are all stored in this backpack. The bigger the backpack, the more it could hold. Sometimes the backpack gets very messy because there is so much stuff in it so you would want to "defragment" it. What this does is it organizes everything in the backpack so it is easier to find stuff later. Hard drives are measured in GB and TB. 1000GB = 1TB.
Video Card: A video card is like the eyes of a person with very bad eyesight. It lets you see everything that is happening but it needs eyeglasses(monitor) to show you. Without the video card, you can't see what you're doing. The video card requires a lot of power and when playing video games and it can get very hot.
Monitor: The monitor is like eyeglasses. It lets you see everything that is happening. Without it, you can't see anything. The monitor is connected to the video card.
Network: The network is the persons phone, it can be a wired house phone or a wireless cell phone. It is how the person communicates to other people. How fast the person can talk to others depends on how fast the internet is.