Gradients
Gradients is a term originating from painting, where colors are laid out in progressive transitions from one another.
Gradients are best used in combination with a mask to apply the effect from any tool in one area of the image and then transition seamlessly out of it.
Gradients shapes
The basic gradient is simply a linear transition from opaque (black) to transparent (white), although it is possible to define more complex gradients such a black - white - black, or circular shapes surrounding one another.
When to use: Use a gradient layer mask over a lighting effect to simulate or accentuate light sources. You can also use a gradient mask in combination with light tools to achieve a HDR look, for instance if the sky is too dark but you don’t want your brightening effect applied on the ground. Generally speaking, using a gradient mask is a quick way to achieve a seamless transition when your effect is applied selectively on a large part of the image.