PC Parts Distinction Help

So, amateur PC builder here. I have already built a PC for a friend, but I am still pretty blank on what is the distinction between "good parts" and "bad parts". For example, I know that a 3.7 GHz quad core Intel with turbo charge capability is good, but I don't know what separates that from thousands of similar processors. I have absolutely what makes a good graphics card, besides having the work "titan" beside it. What makes a good mother board, what makes good ram (besides more of it), etc. My end goal is to actually know how to make my own pc builds without asking for help on what is good and what is not.