Collections
From Shopify Wiki
The liquid variable collections contains a list of all the collections this shop uses.
You can access collections by calling their handle.
Example usage:
{% for product in collections.frontpage.products %}
{{ product.title }}
{% endfor %}
You can also use an indexed-style access, using either a literal or a variable:
{% for product in collections['frontpage'].products %}
{{ product.title }}
{% endfor %}
-- or --
{% assign collectionName = 'frontpage' %}
{% for product in collections[collectionName].products %}
{{ product.title }}
{% endfor %}
This would print the names of all products that are in the collection with handle frontpage