Normally, main index template file, index.php, consists of get_header();, posts (the loop) and get_footer();.
If footer.php file exists, the code inside this file will appear in get_footer(); section. In footer.php, you might want to show WordPress support by having the "powered by" link in this file. I also include the WordPress version, so that the reader will know what version of WordPress I am using for this web site. Lastly, don't forget about wp_footer();. It has the same purpose as wp_head();. It will hook up with any functions that linked to wp_footer, especially from other WordPress plugin.
- WordPress Theme Development 1 - Start with Theme Style Sheet
- WordPress Theme Development 2 - Minimum Main Index Template
- WordPress Theme Development 3 - Header