CSS Floats
From the book: Floats, page 96
Floats are perhaps one of the most important CSS rules, as they allow you to stack content horizontally as well as vertically. Generally, HTML by default stacks content downwards and, without the float rules, it was very difficult to get content to sit side-by-side.
Both objects migrate to the left of the page, stacking from the top-down.
Soups floated left, and milkshakes floated right.
When both (or multiple) objects are floated left, they stack side by side.