By default, any new texture layer added to the shader tree will modulate the 'diffuse color' channel, meaning the layer will affect the apparent visual color of the surface. Should you wish any layer to effect the surface differently, such as applying a bump or displacement map, this is specified via the 'effect' column of the shader tree. A layer can be set to modulate a specific effect, by way of a RMB click over the name in the effect column and selecting a new type from the pop up contextual menu. Layers can also be added and effect other items, such as changing the function of a render output, or using procedural textures to modulate a volumetric lights intensity.
Depending on the type of layer selected, different contextual menus will open, revealing different options. Each is fully covered on their respective document pages, click the titles below to view the associated documentation pages.