Length
This is sometimes referred to as MOL, or "Maximum Overall Length". 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.
Width
Width refers to the complete diameter of the widest point of the light bulb.
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 Lamps
This refers to the number of Fluorescent or HID light bulbs that this particular ballast can light.
Bulb Type Used in Ballast
This refers to the style of light bulb that this particular ballast can light.