When you float an element it becomes a block box. This box can then be shifted to the left or right on the current line.
Knowledge Base
HTML – CSS Float – Introduction
First we need to understand a number of basic important concepts and definitions that determine how web page content is rendered.
WordPress: Display the Primary Sidebar only for certain menu options
To only display the Primary Sidebar for certain menu options, you can change this:
sidebar.php
... <?php dynamic_sidebar('sidebar-primary'); ?> ...
…into this:
... <?php if (is_category('mycategory')) { ?> <?php dynamic_sidebar('sidebar-primary'); ?> <? } ?> ...
What this does is as follows: if you have a menu option that shows all posts in category “mycategory”, the Primary Sidebar will be displayed. For all other menu options, the Primary Sidebar will not be displayed.