Can I get a "base URL" in WordPress within a template file?

You can try using

<?php echo home_url(); ?>

By using this can get site url like www.xyz.com

<?php echo home_url('/contact'); ?>

By using this syntax you will get url like www.xyz.com/contact

Yes you can get "base URL" with a simple function.

<?php echo get_bloginfo('url') ?>

after that with / you can reach to any page just type the page name.

get_bloginfo('wpurl'); would be the preferred method of getting the base url of your WordPress installation. This always returns the absolute base url for the install where as get_bloginfo('url'); is for the actual blog address of your WordPress install.

Yes, you can use get_bloginfo('url') just like that or define a constant...

define('BASE_URL', get_bloginfo('url'));

If you are working on a template and want the URL fragment to that theme folder, use...

bloginfo('template_directory');