Is there any rule when to use panels and when to use templates in drupal 7 I want to know about the panels and templates use cases as i am confused when to use panels and when to use templates as each page can be designed by both the ways then which is the best solution.
 As we can easily layout both the things through html css.
Simple rule:


If you want to add some other blocks, content panes, or other information that is related but not included in content; you need to use Panels. 
If you want to display the content in a custom way and all information you want to display is already included in content, then you don't need Panels, use a template.