As you learn CSS, it will start to make more sense why it's so important to structure your code in logical way.
For now, you can use the metaphor of moving to a new apartment. You divide your things into boxes because it makes moving easier. The silverware would have its own box, and so would the books. You would also write what's in the box on the top flap. In our metaphor, the id and class attributes are like marking what's inside the box.
He graduated from Ohio University's visual journalism program in 2011, and now works at the design and development firm MojoTech.