When is a motorcycle not a motorcycle?

A motorcycle is a two-wheeled motorized device used for transporting people.

A Segway is a two-wheeled motorized device used for transporting people.

Why does riding a motorcycle require a license, and riding a Segway not?

What are the legal boundaries between a bike, a motorized bike, a Segway, a motorcycle, a motorcycle with a sidecar, and a car?