The "Subpage Default" is your default sub page with sidebar.  The "Style Guide" page has been setup to display your site default styles.  The "Subpage Fullwidth" template displays without the sidebar with the content area at the same width as your default template and the"Subpage Fullwidth Wide" displays with the content area at the wider width.

• The index.php is your home template and assigned automatically to your home page.  This templates operates the same as your full width template.