Transparent Multi-Layer Editing
Using multiple layers being the scenes to keep editing simple
Layers are a very useful and sometimes a fundamental part of editing.  In most editors, when you adjust the brightness, contrast, tint, sharpeness, etc. of your image, this is directly applied to your image when you release the slider (see the section entitled, "Dynamic Editing").
In some editors, including Sagelight, many of the changes you make are applied to your image later.   This is where layers find a lot of their power.
Definition of a Layer
A layer is a way to store multiple changes to your image without applying it to your image.  You can also specify that you only want to change parts of the image but not all of it (i.e. mask data).   Layers allow you to perform multiple actions on your image in realtime and see the results without touching the actual image data.
This is very handy, because you can have a brightness layer, a contrast layer, a tint layer, and so-forth.  Layers are in order.  So, if you have a sequence Brightness->Contrast->Tint, you can then change the Contast layer and the tint layer (being underneath the Contrast Layer) will be re-applied instantly based on the the new contrast.
You can then change the Tint or Brightness layers.  The basic rule is that any layer that is changed, the layers beneath that layer will change as if the above layer (and the layers above it that are applied together) is the editied image.
As much as layers are a powerful tool in editors, manually editing layers is sometimes tedious and hard to understand. 
Sagelight's Layer Technology: How Sagelight Uses Layers
Sagelight is designed to perform most of what you can do by manually editing layers very easily, automatically, and transparently -- much more easily than with standard layers, without ever seeing or dealing with the layer itself.
As discussed in the section, "Dynamic Masking", Sagelight uses many layers in the Quick Edit Mode and Pro Quick Edit Mode (aka Kayak Mode) to allow you to dynamically change your image in many ways.  The Pro Quick Edit Mode, for example, can use up to 20 layers simultaneously, allowing you to make over 50 types of changes to your image without applying it to your image until your ready.  Once the changes are applied to the image, only one change is made, rather than changing your entire image for every slider movement or button press.
An example of flexible layer usage in Sagelight
Sagelight also uses layers invisibly in many other functions.  The Photo Filter / Gradient is a good example.  In editors where you need to create a mask layer to add a gradient, you need to do the following:
  1. Create the mask by hand (through some gradient tool)
  2. Exit the mask editing function and then adjust your image through some tint or brightness controls.
The problem is that you can't change the mask while you're changing your image.  You either have to start over or just accept the result because it's just too much work to change it.  This can lead to work that is acceptable but not optimal.
Sagelight Tone Blender (Multi-Layer Example)
Original Image (Image Credit: "Davos" by Thomas Claveirole)
Imagee Changed in Realtime via Layers
As with many Sagelight functions, the Tone Blender you to simultaneously adjust the mask and the tonal changes to the image at the same time.  This also works in the Quick Edit Mode for general masking.
This gives you much more control.  You don't have to accept the results based on just one mask action. This is because it's easy to controls independently where the Tone Blender manipulates at least 3 different layes.  The Quick Edit Mode and Pro Quick Edit Mode can work with over 20 layers simultaneously; you can mask the results and control all layers through the same mask.
Behind the scenes, Sagelight is using many layers to help you with the toning, contrast, and other functions, to keep it powerful and easy, as well as keeping the image quality very high.
Sagelight uses dynamic layers in many functions, such as the Photo Filter, Vignette, Smart Light, Quick Edit Mode, Pro Quick Edit Mode, Vintage, Brush and Masking modes, etc.
While the idea of layers itself is not new technology, Sagelight has taken the concept of layers to a high level to make editing powerful while keeping the advanced details invisible to the user.