Short definition
A marketing funnel is the staged model — typically TOFU (top of funnel), MOFU (middle), BOFU (bottom) — describing how strangers become qualified leads and customers.
In depth
Funnels are useful as a planning tool: each stage has its own content, channels, conversion benchmarks, and metrics. TOFU is awareness (SEO content, social, ads). MOFU is consideration (comparison pages, lead magnets, email nurture). BOFU is decision (case studies, demos, pricing, sales calls). Modern funnels are non-linear thanks to AI search — buyers may enter at the bottom via an AI citation. Plan content as both a funnel and a network.
Example
An agency maps every existing page to TOFU/MOFU/BOFU and finds 80% of pages target BOFU. They invest in a glossary and blog to fill the top, lifting total leads 3x.