What is a Marketing Funnel?

What is a Marketing Funnel?
A marketing funnel is a marketing term for a consumer-focused marketing model that illustrates the theoretical customer journey from a website visitor for the first time, toward the business goal such as click, registration, download or purchase of a good or service.

The marketing funnel metaphor is relevant because it narrows as it progresses, that is the probability of sales decreases at each phase of the funnel.