@Rachel No worries! It was pretty much all foreign to me before I built my first computer too! And then, unfortunately, I became more familiar with everything when I had to figure out how to fix the issues from my PSU going bad. PSU, by the way is short for Power Supply Unit. GPU is your Graphics Processing Unit (also called Graphics Card) and CPU is your Central Processing Unit (also just called processor).
I'm not a "computer guy" so the best explanation I can give you for the motherboard is that it is basically the "hub" for all the other parts of your computer to communicate with each other. Everything gets plugged into your motherboard- your hard drive, your CPU, your GPU, your PSU, USB ports, speakers, everything. Your computer will come with a motherboard, you will only have to purchase one if something happens like it did to me and your original motherboard needs to be replaced.
As far as your other questions, just from my own experience, it doesn't matter what version of Photoshop you are using- I am still running CS4 on my machine. And again, I can only speak from my experience, but I have found laptops to run much less efficiently than desktop units for this type of work. I'm sure you could probably build a laptop that would compete...but I've always been wary of the life expectancy of a laptop when you are pushing it with graphics and illustration work, they don't seem to last as long as desktops, and at least to me, upgrading / repairing a desktop seems much easier.