What is the difference between a DODGE W-100 and a W-150?

The W designation indicated four wheel drive (as opposed to the two wheel drive D series). To the best of my knowledge, the W150 designation was assigned with the first generation of Dodge Ram pickups (1981 - 1983), and the W100 name (which was in existence before the Dodge Ram line was introduced) was revived to indicate what was basically a D/W 150 with different trim options.