In simple terms, a Plus User has full, 3D functionality in Vertex. This means that they can import 3D models and use the 3D Viewer to interact with the models. This includes taking snapshots of the current view to add to stories. Plus Users can also open Vertex Stories in order to collaborate with others in 3D.
A Starter user has 2D functionality in Vertex and can open shared Vertex Stories. They cannot import or view models in the 3D Viewer. When viewing a story, the model appears as 2D (unless they have been given a 3D trial period), and they cannot manipulate the camera view. However, they have all of the features that they need to collaborate effectively on a story, such as adding comments.
For more information on what both types of users can do in a collaborative story, see What can collaborators do in a story?