Omnidirectional (omni) lights are standard light objects that provide a point source of illumination that shoots out in all directions. They're easy to set up, but you can't restrict the focus of their beam. You can, however restrict which objects are illuminated by an omni light.
Above: Top view of an omni light
Below: Perspective view of the same light
Above: Top view of an omni light
Below: Perspective view of the same light
When no lights exist in your scene, two invisible omni lights are turned on by default to provide overall illumination in the scene. However, as soon as you create your own light in the scene, the two default lights are turned off.
You can apply attenuation to omni lights.