A template tag is a code that instructs WordPress to “do” or “get” something. Like in the header.php, we use the tag bloginfo ( ‘name’ ) to get “Site Title” from the wp-options table which is set in Setting > General in WordPress admin.
The the_title() template tag is used to display the post title.
wp_list_cats() is to display categories.
get_header() for getting header.
get_sidebar() to display the sidebar on page.
get_footer() to get the footer content on page.