So far, we focused on the differences between tiled and untiled, but you also have some differences coming from the source property.
So, how can we define a source? What is its purpose?
Sources in OpenLayers 3 language define how and where you can access the layer. You always need a source from an ol.source subclass in order to retrieve and display layers.
Why did OpenLayers 3 development team invent this concept?
It was mainly for the organization of the code and reusability of the same principles. You gain modularity because you have only three types of layers but more sources. It enables you to decouple the type of layer you are using from the data source and the way you call it. It's simpler to manage a new source than to create a new layer by example if a new source appears.
You can see in the following diagram that ol.source.Source has three main child subclasses, ol.source.Vector, ol.source.TileSource, and ol.source.ImageSource.