HTML Dog has probably been the most useful tool in this whole ordeal for me. Here, in the intermediate tutorial, Patrick Griffiths explains the difference from the ID and the div tags. ID tag's identify a particular rule, whereas divs describe classes. Though not vital information, this can probably help alleviate some frustration, and for that, it's worth checkint out.
This is an awesome site that breaks down the science of each css tool. It gives a simplified explanation and easy to understand examples of how your code will change the visuals of your page. I will use this for reference in the future.