CSS Fundamentals: Containing Children - Nettuts+
http://net.tutsplus.com/tutorials/html-css-techniques/css-fudamentals-containing-children/

I've received multiple requests for simpler CSS tutorials that teach the tricky fundamentals. This will serve as the first entry in a series that will receive new additions sporadically each month. Today, we'll be reviewing the overflow: hidden, and clearfix tricks to force a parent div to contains its children.

net.tutsplus.com: css fundamentals