BRDF Type - Determines the type of BRDF (the shape of the highlight). See the BRDF example.

Phong - Phong highlight/reflections
Blinn - Blinn highlight/reflections
Ward - Ward highlight/reflections 
GGX - GGX Microfacet highlight/reflections

Reflection Color - The reflection color. Note that the reflection color dims the diffuse surface color. A node can also be used to control this property. See the Reflection Color example.

Glossiness - Controls the sharpness of reflections. A value of 1.0 means perfect mirror-like reflection; lower values produce blurry or glossy reflections. A node can also be used to control this property. See the Reflection Glossiness example.

GGX Tail Falloff – Controls the transition from highlighted areas to non-highlighted areas when the BRDF Type is set to GGX.

Metalness – Controls the reflection model of the material from dielectric (metalness 0.0) to metallic (metalness 1.0). Note that intermediate values between 0.0 and 1.0 do not correspond to any physical material. This parameter can be used with PBR setups coming from other applications. The reflection color should typically be set to white for real-world metals. A node can also be used to control this property.

Use Roughness – This option controls how Glossiness is interpreted. When Use Roughness is selected, the Glossiness inverse value is used. For example, if Glossiness is set to 1.0 and Use Roughness is selected, this results in diffuse shading. Conversely, if Glossiness is set to 0.0 and Use Roughness is selected, this results in sharp reflection highlights. 

Slide to change the BRDF type.

Example: Reflection Color

This example demonstrates how the Reflection Color parameter controls the reflectivity of the material. Note that this Color also acts as a filter for the Diffuse Color (e.g., stronger reflections dim the diffuse component).

Example: Reflection Glossiness

This example demonstrates how the Glossiness parameter controls the highlights and reflection blurriness of the material. For this example, we have used the HSV color range, but the same results can be achieved with these values and the RGB color range. Fresnel IOR = 3.5.

Fresnel Reflections – Enabling this option makes the reflection strength dependent on the viewing angle of the surface. Some materials in nature (glass etc) reflect light in this manner. Note that the Fresnel effect depends on the index of refraction as well. See the Fresnel example.

Lock Fresnel IOR – Allows you to unlock the Fresnel IOR parameter for finer control over the reflections.

Fresnel IOR –  The Index of reflection to use when calculating Fresnel reflections. Normally, this is locked to the Refraction IOR parameter, but you can unlock it for finer control. A node can also be used to control this property.

Anisotropy(-1...1) – Determines the shape of the highlight. A value of 0.0 means isotropic highlights. Negative and positive values simulate 'brushed' surfaces. The accepted values are in the range from 0.99 to -0.99. See the example.

Anisotropy Rotation – Determines the orientation of the anisotropic effect in a float value between 0.0 and 1.0 (where 0.0 is 0 degrees and 1.0 is 360 degrees). See the example.

UV Vectors Derivation – Specifies the method for deriving anisotropy axes:

Local object axis – Uses a local axis for the anisotropy effect. 
Specified uvw generator – Allows the user to connect a Mapping node for the anisotropy effect.

Anisotropy Axis – Specifies a local object axis for the anisotropy effect when UV Vectors Derivation is set to Local object axis.

Enable Thin Film – Toggles the thin film effect on and off. When disabled, all other parameters in this rollout are inactive.

Thickness Min (nm) – Determines the minimum thickness of the thin film in nanometers. See the example.

Thickness Max (nm) – Determines the maximum thickness of the thin film in nanometers when a texture is assigned to the Thickness Blend slot. If there is no texture map, only the Min thickness value is used.

Thickness Blend – A map used to blend between the Min and Max Thickness parameters. It is expected to be in the range of 0-1. A node can also be used to control this property.

Thin Film IOR – Specifies the reflective index of the thin film. A map can be attached to this slot. A node can also be used to control this property.

Trace Reflections – When disabled, reflections are not traced even if the reflection color is greater than black. This can be disabled to produce only highlights. Note that when disabling this parameter, the diffuse color is not dimmed by the reflection color, as would happen normally.

Max Depth – The number of times a ray can be reflected. Scenes with lots of reflective and refractive surfaces may require higher values to look correct.

Enable Dim Distance – Enables the Dim Distance parameter, which allows you to stop tracing reflection rays after a certain distance.

Dim Distance – Specifies a distance after which the reflection rays are not traced, when Enable Dim Distance is active.

Dim Fall-off – A fall-off radius for the dim distance, when Enable Dim Distance is active.

Reflect On Back Side – When enabled, reflections are computed for back-facing surfaces, too. Note that this affects total internal reflections, too (when refractions are computed).

Soften – Softens the edge of the BRDF at light/shadow transitions.

Affect Channels – Allows you to specify which channels are going to be affected by the reflectivity of the material.

Color only – The reflectivity affects only the RGB channel of the final render.
Color+alpha – Causes the material to transmit the alpha of the reflected objects instead of displaying an opaque alpha.
All channels – All channels and render elements are affected by the reflectivity of the material.