This is a general question and something that dawned on me and seemed to make sense. I have seen many people use clearing divs and
Both methods are old fashioned. The latest "trick" is to use overflow property for the container of float elements.
If for example you have:
text here
text here
with float class float:left then it's better to use overflow:hidden or overflow:auto than or the hr method.
Demo: http://jsfiddle.net/vALSL/
Read more here: http://www.quirksmode.org/css/clearing.html