The search_form is a simple tag that creates a form that could be used with the search tag.
<cms:search_form msg='Search' processor="<cms:show k_site_link/>search.php" />
This parameter sets the message displayed within the search textbox. By default the string 'Search..' gets displayed.
This parameter sets the action parameter of the generated form. This needs to be set to the URL of the page that has the pages tag on it.
If left empty, the pages tag is expected to be on the same page as the search_form tag.
This tag sets no variables of its own.