A sales funnel is the path that takes a person from 'I don't know you' to 'I buy from you', built in measurable stages. That's it. The name describes the shape: many enter, few reach the bottom, and the job is to lose as few people as possible at every step.
The stages of a marketing funnel, without acronyms: at the top are people who have the problem but don't know you (they need attention), in the middle are people who noticed you but don't trust you yet (they need value), at the bottom are people ready to talk (they need a clear invitation). Each stage has one job, one piece of content and one number to watch.
One complete real example, with lifetime Ads Manager numbers. For AsteNow, a real-estate auction platform, I built the entire funnel. Top: a quiz that produced 669 leads at €0.90 each, with a 4.38% CTR. Middle: 1,511 completed registrations at €0.42 each. Bottom: retargeting with 5,230 landing page views at €0.066 each. Three stages, three campaigns, one engine.
How to build a sales funnel, in 5 steps. One: pick one problem and one audience (a funnel for everyone converts no one). Two: build the entry offer, the reason to leave you a contact (a quiz, a guide, a free analysis). Three: one page with a single possible action. Four: the follow-up sequence for those who don't act immediately (3-5 emails that educate and lead back to the action). Five: measure every step and fix the one where you lose the most.
The mistakes I see most often: sending traffic to the home page (it's a lobby, not a path), asking for the call from someone who just met you (like proposing marriage on the first coffee), and having no follow-up (90% of contacts aren't ready today, but some will be in a month: without a sequence, you're gifting that month to a competitor).
What it costs to build: less than not having one. Every month without a funnel, your ad budget buys visits that evaporate and word of mouth stays your only channel. With a funnel, every visit has a path and every contact a measurable destiny.
If you want to see where yours leaks, this site has a 2-minute audit that analyzes the 4 layers of your acquisition engine and tells you which one to start from.