The "Subpage Default" is your default sub page and displays with a right sidebar.  The "Subpage Fullwidth" displays the content at full width without a right sidebar.  The "Style Guide" page has been setup to display your site default styles.

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

The page will display the page title by default in the content area.  The checkbox "Hide Page Title" removes the title from the page.