Length
This is sometimes referred to as MOL, or "Maximum Overall Length". For light bulbs, it measures the absolute length of a light bulb, from the top of the bulb to the bottom of the base and is typically expressed in inches. For light fixtures and ballasts, it applies to the complete vertical length of the item, out of the package.
Width
For light bulbs, the width refers to the complete diameter of the widest point of the light bulb. For light fixtures, it applies to the widest horizontal point of the fixture, out of the package.
Beam Angle
The beam angle refers to the width of the light beam coming from a reflector light bulb. The higher the number, the wider the beam spread.
# of Lights
This refers to the number of lights on a Christmas string set or in a pack of replacement bulbs.
Bulb Type Used in Ballast
This refers to the style of light bulb that this particular ballast can light.