Grid Systems, Skeleton and Bootstrap
When we have a display device larger than a phone, we often want to switch to a different layout, with different columns and such. For example, we might have a nav bar on the left and a twitter feed to the right in addition to a wide column of main content in the center. We might even have multiple columns of content. To make things neat and orderly, we often have a layout based on a grid. (Typically, you'll see the grid based on 960px, not because that amount matches any device size, but because 960 is divisible in lots of ways.)
For an idea of the theory, please read the Mozilla Developer Network article on grids Fair warning, though, it is long. Stop when you get to Line-based Placement. Our goal is to learn the basic concept and some of the techniques, but not every detail.
In this class, I'm using Skeleton which I like a lot because it's simple and well-documented. However, I acknowledge that many people like Bootstrap, and you're welcome to use that, too. The MDN article used to mention both Skeleton and Bootstrap; but it's been revised.