groups = layers = types = categories
Something I was researching two years ago today but forgot to draw it out. It was very eye opening to realize that they are the same because they certainly look different and are used differently.
Example: How is a layer in a paint program a group? A layer contains a group of the items that are on that layer. But what if you select multiple layers? That becomes another group, which can be turned into a layer because it is a layer, just oriented differently.