What is a Service?

How do services differ from goods?

Units: Goods are measured in physical units, while services often cannot be given an easy-to-understand unit.
Variability: Goods are typically mass-produced, while services are usually custom-produced.
Complexity: Goods are mass-produced, and considered as units, but services however are typically defined by rather complex contracts.
Simultaneity: Goods can be stored, but services must be produced and consumed at the same time.