What are collections
Collections are custom content types different from pages and posts. You might create a collection if you want to treat a specific set of articles in a unique way, such as with a custom layout or listing. For more detail on collections, see Ben Balter's explanation of collections here.
Create a collection
To create a collection, add the following in your configuration file:
collections:
tooltips:
output: true
In this example, "tooltips"" is the name of the collection.
Interacting with collections
You can interact with collections by using the site.collectionname namespace, where collectionname is what you've configured. In this case, if I wanted to loop through all tooltips, I would use site.tooltips instead of site.pages or site.posts.
See Collections in the Jekyll documentation for more information.
How to use collections
I haven't found a huge use for collections in normal documentation. However, I did find a use for collections in generating a tooltip file that would be used for delivering tooltips to a user interface from text files in the documentation. See for details.