In it's simplest form - a sales funnel is what converts individuals into customers.
How is this done?
If you picture a funnel, the top is very wide - and this represents the start of the funnel. The idea is to cast a wide net through various forms of advertising targeted towards what the business believes is ideal customers ... whether social media, content marketing, PPC, direct mail, phone calls, etc
Some folks will be "captured" and continue down the funnel, while others will "fall out" ... hence the narrowing of the funnel. What do I mean by captured? Normally, this means an email address; however, it can also reference mailing info, phone info, social media info, etc. Basically, a way to keep in touch with what is now called a lead or a prospect.
Up next comes "lead nurturing". This is where the business attempts to form a relationship with the lead by sharing relevant information, keeping in touch, answering/asking questions, etc. Sometimes, even ... this can mean taking the lead out to dinner or sending them on vacation.
For example, my parents just got a new investment guy ... and since my parents are at retirement age, they're worth a fair amount to an investment guy, so it's worth that individual to take my parents out to eat, send them on little trips in order to try and "win" them over. So, those meals were part of that guys sales funnel. And, I don't know free food and trips was what did the trick; however, my parents went with him.
My point is, a sales funnel can go beyond simply sending people a bunch of emails.
After the lead has been nurtured, it's time to close the sale - turning the lead into a paying customer.
Most of your "typical" sales funnel definitions end there; however, I do want to take this sales funnel definition a step further because, it's not over once you make one sale. You now have a paying customer, who has enough trust in your business to give it money. Now, you need to keep them happy. Keep them comfortable. Keep them informed. And, of course, sell them more products and services!
Don't forget that ... there's a life after the traditional sales funnel, and someone is going to need to maintain that aspect. And, typically, it's value is worth way more than the initial sales funnel.