Schema markup (structured data) is code added to a website's HTML to describe content in a standardized format that search engines, voice assistants, and other bots understand. The most common formats are JSON-LD, Microdata, and RDFa. Schema.org provides a shared vocabulary—you mark up a product with properties like name, price, availability, rating, and Google can then display a rich snippet directly in search results. Schema markup is invisible to users but transforms plain-text search results into interactive cards. An e-commerce site selling shoes might markup a product with JSON-LD; Google shows the product image, price, star rating, and "Add to Cart" button right in the SERP, increasing click-through rate dramatically.