Since WordPress 3.8 nice menu icons on dashboard appear beside different settings menu e.g. Posts, Pages, Media, Appearance etc. These icons do not use images but are build with CSS. If you are a WordPress plugin or theme developer, you can use these icons to have a nice icon for your product’s dashboard menu.
Admin menu items icons can only be added with register_post_type() and add_menu_page(). Both have an option to set an icon.
Read Add Dashicons to your WordPress Plugin Admin Menu Icon