Zoe Gillenwater has an excellent article,
Curing Float Drops and Wraps, at the Nemesis Project. “Floats are a great way to create layouts with multiple columns: they allow blocks to be taken out of the normal flow, where they would just be stacked top to bottom, and positioned beside each other instead. Unfortunately, however, you’ll occasionally run into a situation where one block drops down below the other one instead of sitting side-by-side as you anticipated. This is commonly referred to as a float drop.”