Collections
A Collection is a container whose children are categories pulled from a module — a blog, a Datatable, or another category-driven module — tied to a page template that generates a public detail page for every category automatically, instead of an administrator building a category detail page by hand for each one.

When to Use a Collection
Use a Collection when a module's categories (for example, blog categories, or departments/parks/program areas on a Datatable) should each get their own public landing/detail page with a consistent layout — all sharing the same design but pulling in that category's own entries.
Note
If a generated category page looks wrong, check the Collection's Page Template first — it's a shared layout, so a fix there applies to every category the Collection covers.