The marketing funnel depicts the steps of a hypothetical buyer through his decision-making process. The funnel is widest at the top and then gradually grows more narrow. The earliest models depicted a customer entering the funnel as a novice and then sliding down the funnel and through the steps of awareness, interest, desire and action, meaning a purchase.
If you're wondering what a sales funnel is, simply imagine a real-world funnel. At the top of that funnel, some substance is poured in, which filters down towards one finite destination. In sales, something similar occurs. At the top, lots of visitors arrive who may enter your funnel. However, unlike the real-world funnel, not all who enter the sales funnel will reemerge out from the other end.