Loop template files are used to format the loop based on what type of page you are visiting.
You can have loop templates for different loops. For example: loop-archive.php or loop-category.php which represent different loops for archives and categories, respectively.
You can call a loop from inside one of you page templates like this:
<?php /* Run the loop to output the posts. * If you want to overload this in a child theme then include a file * called loop-index.php and that will be used instead. */ get_template_part( 'loop', 'category' ); ?>
Using this method will call the file loop-category.php and run the content within in it. Inside loop-category.php you might have code to format the loop just for category pages.
Template Hierarchy: Category Templates, Tag Templates, Taxonomy Templates, Page Templates, Post Type Templates, Author Templates, Date Templates, Search Templates, 404 Templates, Attachment Templates, Loop Templates