app/views/layout/application.html.erb this is a common layout in which you will found <%= yield %>
which render all the pages in <body>
tag. Now as per your requirement you want some common template to show on all pages.
So better to make one partial
file..For example, Header
and Footer
remains same in whole site. For doing this, make one partial file called _header.html.erb for header part and _footer.html.erb for footer part. Put these files under app/views/layout/_your_partialfile.html.erb
Then render them like:
<%= render partial: "/layouts/header" %>
<%= yield %>
<%= render partial: "/layouts/footer" %>
For more info refer : http://guides.rubyonrails.org/layouts_and_rendering.html
I hope this makes you clear to understand now. :)