The sales funnel is a gigantic filtration system. In the top of the funnel is every person who ever comes in contact with your ideas. Everyone who finds you from a Google search, everyone who hears about you from a colleague, or sees a retweet about your product, or is exposed to your advertisement at stack overflow or through any other method. All of these entrants to the funnel are "unqualified sales prospects". As we move further down the funnel we find less people (as most prospects drop off) but we find the remaining prospects are more "qualified". These are people who are more interested in what you are selling, people who are more engaged by your content, people who are more likely to continue toward a sale. Finally at the bottom of the funnel we have a tiny trickle of actual customers. Or, perhaps, a roaring torrent of customers!
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