I have switched over to full CSS in the last six months, so still have a lot of learning ahead of me. From my understanding to date, you use a Class tag for a repeated Div assignment and an ID for a unique assignment. So in other words, if you have a font style, or div container style, which appears numerous times on a single page, use "< div class="example" > ... and if it's a unique container, with a single use per page, use "< div id="example" >
Hope this helps?
All the best,