The "Subpage Default" is your default sub page with a sidebar and the "Subpage Fullwidth" displays the content area the full width of the container.  The "Style Guide" page has been setup to display your site default styles.  The "Subpage Fullwidth Share" displays the full width template with the share option below the last full width section.

*NOTE - The index.php is your home template and assigned automatically to your home page.  This templates operates the same as your "Subpage Fullwidth".