A render channel is a named description of image data with a given data type. Its framebuffer specifies where and how the channel's image data should be written out.
In more practical terms, render channels are a mechanism for outputting multiple images, each containing different information, from a single pass. When you render the pass, you can specify which channels should be output in addition to the full pass.
By default a Main render channel is declared for every pass (you can think of it as the "beauty" channel rendered for each pass) and it has a color data type. In addition to the Main render channel, there are other built-in render channels for outputting depth, motion vectors, normal vectors, labels, and pixel coverage data.
Preset render channels allow you to isolate scene attributes that are commonly rendered in separate passes. All you need is your default pass and you can quickly output the preset ambient, diffuse, specular, reflection, refraction, and irradiance render channels.
