Another useful concept is to identify Customers vs Users – those who will pay for your idea versus those who may just find it useful. Particularly if you have multiple facets to your product or service. Facebook for example has users (you and I, with our profiles) and customers (businesses who advertise). Users might often be customers-in-waiting.
Spotify is a fascinating case study of a business that has both users and customers and, through astute observation of their user’s behaviour, have been very successful at converting free users to paying customers.