The standard shader node is a multi-purpose color shader capable of producing all type of materials, from simple plastic (Phong), to things like car paint and skin (with effects like sub-surface scattering and Fresnel). The node outputs a color value (RGB).
Due to the large number of controls, the standard shader is split up into several groups:
The standard shader is very powerful, and allows a large number of different sorts of materials to be created, but can be somewhat daunting at first. It's often best to start with a material which has been pre-defined using the standard shader and make incremental changes to it to get the effect you want, rather than starting from scratch. The Arnold Tutorials manual contains a section on using the standard shader to create realistic materials, with a short 'glossary' of settings suitable for common material like car paints, chrome, glossy and matte plastic, and others.
The individual settings for each group (Diffuse, Specular, Reflection, Refraction, Bump Mapping, Sub-Surface Scattering, Emission, Caustics, etc) are described in more detail in the child pages below.