i like to explain divs as say a sticky note(all different sized post-it's) stuck on a piece of paper. if you remember that everything inside a div that you have set say the text color will be the same through out all the stickynotes on/in each one unless you specify otherwise.
i can elaborate more if you like however the best tool to use is to play around with the div and start with background-color and change it for each div you put on the page. then you can see how they relate to one another and itself.
later you can put div inside div inside div inside div and see how that turns out. As far as the line break you speaking of you can change that as well for each div you put on the page..