A sales funnel can be visually illustrated as an inverted pyramid where the biggest portion lies at the top of the aptly-named funnel. Like water pouring through, the current gets ever smaller and smaller until it peters out at the bottom.
In a similar way, a large quantity of general consumers in a market form the head of the funnel. It is up to the marketer to successfully guide the general consumer from awareness of the businesses product/service all the way down to the final purchase and staying with the business (customer loyalty/retention). In between, there are a number of steps that need to be taken prior to purchase including discovery of the product, evaluation of the product, intent to buy the product.
More traditional marketing studies refer to the AIDA model. AIDA is an acronym standing for "Awareness, Interest, Desire and Action." It uses the same funnel shape as the diagram shown above with the main difference being that it is simpler to understand and contains fewer details. Also, there is no interlinking with the sales pipeline that can also be run parellel to the funnel.