Probably best to use the second option I have above (ABSPATH) because it's WordPress aware. And we know it will point to the main directory for your blog (i.e. where wp-blog-header.php and wp-config.php reside). If that's the /wordpress sub-directory in your main web directory, you should use:
<?php include(ABSPATH . '/trackinfo.txt'); ?>
For 1.5, a theme is a collection of php files a.k.a. templates (index.php, comment.php, etc.) and at least one stylesheet (style.css). Some themes include images, as well. So when we say template around here, we really mean one of your php files, whether for a theme or a PHP/WordPress document of your own creation.
Where your trackinfo.txt should be included depends on what it does and where you want it. If it's just a file with some text/html you want to display in say the sidebar, then you'd want to place it somewhere in sidebar.php, or if your theme doesn't have this in the menu/sidebar section of theme's index.php.