Prior to Blender version 2.8, Layers were used to separate objects in a scene. This tag is specifically for questions related to using layers in blender 2.79 or earlier.
Prior to Blender version 2.8, Layers were described in the manual thusly:
3D scenes often become exponentially more confusing as they grow more complex. Sometimes the artist also needs precise control over how individual objects are lit, and does not want lights for one object to affect nearby objects. For this and other reasons below, objects can be placed into one or more “layers”.
In version 2.8 layers were replaced by Collections.
Use this tag for questions about layers in version 2.79 or earlier.