Shaders are more commonly known as shading networks, because they rarely ever include one item (node). What a shader network is, is a shading group which has a surface material connected to it. The shading group engine is what’s actually connected to your model, or, more specifically, the shape node of your model (this is the part that actually contains the geometry information of the model) and the surface material, which is connected to the shading group is the material which will appear on the model. Multiple models can share the same shading group.