It often happens that we need the same set of pages or files in multiple places across our site. Think of blog articles on the blog and on the home page for example. Collections help with not repeating ourselves in such cases and to access commonly used pages or files collections more easily.
A list of pages
A list of users
A list of images/files
Collections can be used in templates and snippets like this:
$kirby->collection("articles"); $kirby->collection("admins"); $kirby->collection("project-covers");
It's always a good idea to provide a collection that is not too specific yet. In general, it's better not to apply pagination to collections for example, or to set a limit. You might want to use the same set of pages in different places, but in one template you need three and in the next you need 10.