Collections were introduced in Blender 2.8 to replace object layers and groups as a scene organization tool. Use this tag for questions about 2.8 or later Collections. Use this tag for questions about using Collections.
Collections were introduced in Blender 2.8 as a combined replacement for the grouping and layer systems for managing objects in scenes. They do not replace bone layers or view layers. Collections are also used in other parts of the system, such as Particle Systems to provide the objects rendered by the particle system.
The introduction to collections has this to say about them:
In Blender, objects are not directly part of the scenes. Instead, they all get stored in a main database (basically the blend-file).
When they are stored in a scene, they are part of a so-called scene collection. So ultimately all the scene objects belong to this special collection.
While the scene collection contains all the Scene’s objects, the user can also make their own collections to better organize these objects.
Use this tag for questions about using Collections to organize objects.
