In this example, the image is taller than the element containing it, and it is floated, so it overflows outside of its container:

Without Clearfix

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus imperdiet, nulla et dictum interdum...

Add the clearfix hack to the containing element, to fix this problem:

With Clearfix

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus imperdiet, nulla et dictum interdum...