Beauty render channels are components of the final 'beauty' render, where each render channel captures all the light from a particular class of light path from the emitter to the camera.

Direct Lighting Channel

This render channel shows direct diffuse lighting.
In other words, it shows light that is emitted from a light source, and then bounces exactly once diffusely off an object
and into the camera.

left image - beauty render. right image - direct lighting channel

Indirect Lighting Channel

This render channel shows indirect diffuse lighting.
In other words, it shows light that bounces around a scene at least once, and then bounces diffusely off an object immediately before hitting the camera.

left image - beauty render. right image - indirect lighting channel

Specular Reflection Lighting Channel

This render channel shows specularly reflected light.
In other words, it shows light that bounces around a scene, and then bounces specularly (e.g. bounces off a smooth surface) off an object immediately before hitting the camera.

right image - Specular reflection channel

Refracted Lighting Channel

This render channel shows refracted light.
In other words, it shows light that bounces around a scene, and then is refracted through a transparent object immediately before hitting the camera.
Glossy transparent and specular materials will contribute to this channel.

right image - refracted lighting channel

Transmitted Lighting Channel

This render channel shows light diffusely transmitted through a surface.
In other words, it shows light that bounces around a scene, and then is diffusely transmitted through a partially or full transparent object immediately before hitting the camera.
Diffuse transmitter and double-sided thin materials will contribute to this channel.

A double-sided thin material. Right image - transmitted lighting channel

Emission Lighting Channel

This render channel shows directly emitted light.
In other words, it shows light that is emitted from a light source, and then directly strikes the camera.

right image - emission lighting channel

SSS Lighting Channel

This render channel shows light that has undergone sub-surface scattering.
In detail - it shows light that is emitted from a light source, bounces around the scene,
undergoes SSS, exits a medium through a transparent material, and then directly strikes the camera.

right image - SSS lighting channel

Participating Media Lighting Channel

This render channel shows light that has undergone participating-media scattering.
In detail - it shows light that is emitted from a light source, bounces around the scene,
undergoes participating media scattering, then directly strikes the camera.

right image - Participating Media lighting channel