One of the most powerful uses of Adobe Photoshop is the ability to composite multiple images into one. This is something that you aren't able to do in Lightroom and it allows photographers to have complete control over their images to a creative or practical end.
The process of compositing images can be done in a number of ways. One of these is to utilize channels to build a mask of the areas you want to hide, and those areas you want to reveal on an image. This is the method we'll look at today.
Masks are a black and white representation of the areas that will be visible and invisible in an image. The saying commonly goes "White is on, black is off," or "Black conceals, white reveals." Both sayings mean the same thing. On a given layer, the areas of the mask that are white will be visible, and the areas that are black will be hidden. Masking allows us to have complete control over a composite.
As I said before, there are many ways to composite images. While we can use Channels to get a good starting place, we can also use tools like the Select and Mask Utility to further refine the masks we use in our composites.
In the tutorial below, I show you how to use Channels to create a mask of the moon and move it into another image. Check it out!