Often, when you use floats (What is a float?) in your design, you have to clear them, otherwise, you will find your floated elements breaking out of their container. The only catch, is that you had to add extra markup (usually a <div>, a <br />, or a <span>) to apply the clear to. Thankfully, there is now a fix for that, that just requires a bit of css, and NO EXTRA markup (excluding a bit of IE hackery).
Simply, use a :after on the container of the floated elements with the clearing properties added to it.