From BlenderWiki

Shader Nodes

BSDF

Diffuse

Lambertian and Oren-Nayar diffuse reflection.

Color input

Color of the surface, or physically speaking, the probability that light is reflected or transmitted for each wavelength.

Roughness input

Surface roughness 0.0 gives standard Lambertian reflection, higher values activate the Oren-Nayar BSDF.

Normal input

Normal used for shading, if nothing is connected the default shading normal will be used.

BSDF output

Diffuse BSDF shader.

Diffuse behaviour
Lambertian Diffuse	Oren Nayar Diffuse

Translucent

Lambertian diffuse transmission.

Color input

Color of the surface, or physically speaking, the probability that light is transmitted for each wavelength.

Normal input

Normal used for shading, if nothing is connected the default shading normal will be used.

BSDF output

Translucent BSDF shader.

Glossy

Glossy reflection with microfacet distribution, used for materials such as metal or mirrors.

Distribution

Microfacet distribution to use. Sharp results in perfectly sharp reflections like a mirror, while Beckmann and GGX can use the Roughness input for blurry reflections.

Color input

Color of the surface, or physically speaking, the probability that light is reflected for each wavelength.

Roughness input

Influences sharpness of the reflection, perfectly sharp at 0.0 and smoother with higher values.

Normal input

Normal used for shading, if nothing is connected the default shading normal will be used.

BSDF output

Glossy BSDF shader.

Rough Glossy behaviour	Sharp Glossy behaviour
A Rough Glossy Material	A Sharp Glossy Material

Anisotropic

Anisotropic glossy reflection, with separate control over U and V direction roughness. The tangent used for shading is derived from the active UV map. If no UV map is available, they are automatically generated using on a sphere mapping based on the mesh bounding box.

Color input

Color of the surface, or physically speaking, the probability that light is reflected for each wavelength.

Roughness input

Sharpness of the reflection, perfectly sharp at 0.0 and smoother with higher values.

Anisotropy input

Amount of anisotropy in the reflection, 0.0 gives round highlight. Higher values give elongated highlights orthogonal to the tangent direction, negative values give highlights shaped along the tangent direction.

Rotation input

Rotation of the anisotropic tangent direction. Value 0.0 equals 0° rotation, 0.25 equals 90° and 1.0 equals 360° = 0°. This can be used to texture the tangent direction.

Normal input

Normal used for shading, if nothing is connected the default shading normal will be used.

Tangent input

Tangent used for shading, if nothing is connected the default shading tangent will be used.

BSDF output

Anisotropic glossy BSDF shader.

Transparent

Transparent BSDF without refraction, passing straight through the surface, as if there was no geometry there. Useful with alpha maps for example. This shader affects light paths somewhat differently than other BSDF's. Note that only pure white transparent shaders are completely transparent.

Color input

Color of the surface, or physically speaking, the probability for each wavelength that light is blocked or passes straight through the surface.

BSDF output

Transparent BSDF shader.

Transparent behaviour
Transparent Shader (pure white)	Transparent Shader (grey)

Glass

Glass like shader mixing refraction and reflection at grazing angles. Like the transparent shader, only pure white will make it transparent. The glass shader tends to cause noise due to caustics. Since the Cycles path tracing integrator is not very good at rendering caustics, it helps to combine this with a transparent shader for shadows, for more details see here.

Distribution

Microfacet distribution to use. Sharp results in perfectly sharp refractions like clear glass, while Beckmann and GGX can use the Roughness input for rough glass.

Color input

Color of the surface, or physically speaking, the probability that light is transmitted for each wavelength.

Roughness input

Influences sharpness of the refraction, perfectly sharp at 0.0 and smoother with higher values.

IOR input

Index of refraction defining how much the ray changes direction. At 1.0 rays pass straight through like transparent, higher values give more refraction.

Normal input

Normal used for shading, if nothing is connected the default shading normal will be used.

BSDF output

Glass BSDF shader.

Sharp Glass behaviour	Rough Glass behaviour
A Sharp Glass Material	A Rough Glass Material

Refraction

Glossy refraction with sharp or microfacet distribution, used for materials that transmit light. For best results this node should be considered as a building block and not be used on its own, but rather mixed with a glossy node using a fresnel factor. Otherwise it will give quite dark results at the edges for glossy refraction.

Distribution

Microfacet distribution to use. Sharp results in perfectly sharp refractions, while Beckmann and GGX can use the Roughness input for blurry refractions.

Color input

Color of the surface, or physically speaking, the probability that light is refracted for each wavelength.

Roughness input

Influences sharpness of the refraction, perfectly sharp at 0.0 and smoother with higher values.

Normal input

Normal used for shading, if nothing is connected the default shading normal will be used.

BSDF output

Glossy BSDF shader.

Velvet

Velvet reflection shader for materials such as cloth.

Color input

Color of the surface, or physically speaking, the probability that light is reflected for each wavelength.

Sigma input

no description yet

Normal input

Normal used for shading, if nothing is connected the default shading normal will be used.

BSDF output

Velvet BSDF shader.

BSSRDF

Subsurface Scattering

Simple subsurface multiple scattering, for materials such as skin, wax, marble, milk and others. For these materials, rather than light being reflect directly off the surface, it will penetrate the surface and bounce around internally before getting absorbed or leaving the surface at a nearby point.

How far the color scatters on average can configured per RGB color channel. For example for skin red colors will scatter further which gives distinctive red colored shadows, and a soft appearance.

Color input

Color of the surface, or physically speaking, the probability that light is reflected for each wavelength.

Scale input

Global scale factor for the scattering radius.

Radius input

Scattering radius for the each RGB color channel.

Normal input

Normal used for shading, if nothing is connected the default shading normal will be used.

BSSRDF output

BSSRDF shader.

Emission

Lambertian emission, to be used for the material and lamp surface outputs.

Color input

Color of the emitted light.

Strength input

Strength of the emitted light. For point and area lamps the unit is Watts. For materials a value of 1.0 will ensure the object in the image will have the exact same color as the Color input, i.e. make it 'shadeless'.

Normal input

Normal used for shading, if nothing is connected the default shading normal will be used.

Emission output

Emission shader.

Background

Background light emission. This node should only be used for the world surface output, it will be ignored in other cases.

Color input

Color of the emitted light.

Strength input

Strength of the emitted light.

Background output

Background shader.

Holdout

A holdout shader is useful for compositing, to create a "hole" in the image with zero alpha transparency where the object with this shader is located.

Holdout output

Holdout shader.

Ambient Occlusion

The ambient occlusion node gives per material control for the amount of AO. When AO is enabled in the world, it affects all diffuse BSDF's in the scene. With this option it's possible to let only some materials be affected by AO, or to let it influence some materials more or less than others.

Color input

surface reflection color.

AO output

Ambient Occlusion shader.

Mix and Add

Mix or add shaders together. Mixing can be used for material layering, where the Fac input may for example be connected to a Blend Weight node.

Shader inputs

Shaders to mix, such that incoming rays hit either with the specified probability in the Fac socket.

Fac input

Blend weight to use for mixing two shaders, at zero it uses the first shader entirely and at one the second shader.

Shader output

Mixed shader.