Your Light Set includes all real-time and image-based lights in your scene
The Light Set library contains variety of lighting templates you can quickly apply to your scene.
You can also add light sources individually to build a custom light set, which you can save to your library.
Individual lights can be added using the lightbulb button in the tools menu.
Light options include:
Ambient Light: Foods even light across the scene. Envelopes the entire scene, cannot be rotated.
Direct Light: Can be adjusted to point in the desired direction. Mimics the Sun.
Area Light: Casts light within the bounds of a defined, rectangular area. Similar to a Photographers Soft Box lights.
Point Light: Single point light source such as a light bulb.
Spot Light: Casts a smaller ring of light in a specific direction, which can be controlled by rotating and moving the light.
The colour and intensity of lights can be modified in the properties panel when selected.
Image-based lighting can also be used to add environmental light to your scene, which you can read about here.