A page can have a single liquid page module attached to it. When you add the page module to a page, it will pull in all available liquid partials that you have in your template. On the design tab for your site, you can add more modules under the "page modules" tab.
Returns an array of modules attached to the page.
{% if page.modules contains "blog" %}
The page module called _blog.liquid is attached to this page!
{% endif %}
{% if page.parent.modules contains "blog" %}
Do something special just for blog posts.
{% endif %}