Seeing that you don't know very much about computers, i'll try to explain something.
First of all:
The motherboard is the main board, where you will plug all the other things in, like the video card, the ram and so on.

That big green board on the back is the motherboard.
Then:
-The Processor (CPU - Central Processing Unit) is the part that elaborates all the informations, and then sends them to the various other cards (video card, sound card)
-The Hard Disk: is where all the data, files and folders of your PC are saved permanently. This means that when you turn off the PC the data will still be saved
-The RAM (Random Access Memory) this kind of memory is much faster, and is used to elaborate temporary information. When you turn off the PC the RAM will reset.
-The Video Card (GPU) will elaborate all the visive information, and then send it to the monitor, so that you can actually see it. A good Processor is needed to help the video card elaborate the data.
-The sound card: same as the video card, but is used to create audio.
That are the basics, so you can understand a little bit better how a PC works.
I hope I could help you