on enables/disables light source.

visible makes light source visible/invisible. Invisible light source still emits light.
targeted if on, light source direction is set by target object.

target distance is used to set target object position when targted is set.

size x / size y set light source size. Plane shape uses size x and size y values. Disk and sphere shapes use only size x value.

shape sets light source geometry. It can be plane, disk or sphere or IES geometry.

power sets light source intensity per square unit.

Light color can be set through direct color input, temperature or texture.
directional turns light to spot.

directional angle sets spot angle.

exclude/include can include/exclude objects to/from certain light source. 

IES option loads ies data from a file and uses for lighting distribution.  

IES gamma sets which gamma to use while loading ies image.  

IES min value limits minimal emission power to avoid black looking light source from some angles.  

IES roation allows to change ies direction without rotating light object.