The pillar-cluster model emerged in 2017 as a response to Google moving from keyword-by-keyword matching to topic-based ranking. The pillar covers the broad topic at depth (e.g. 'Email Marketing'); each cluster page covers a narrow sub-topic (e.g. 'How to write subject lines', 'Email A/B testing').
Done well, pillar pages can outperform standalone long-form posts because the internal-link structure reinforces topical relevance across the whole cluster. Done badly — pillar pages stuffed with thin cluster links — they create the opposite signal.
Pillar pages typically run 2,000–5,000 words. Cluster pages run 800–1,500. Every cluster page links back to the pillar with descriptive anchor text; the pillar links out to every cluster page with descriptive sections.