Short definition
Schema markup is structured code added to a webpage that tells Google, Bing, and AI engines exactly what the page is about — a service, a review, a person, an FAQ.
In depth
Schema markup is a vocabulary maintained at schema.org and embedded in pages as JSON-LD. Search engines use it to understand entities and relationships rather than guessing from raw text. Pages with proper schema are more likely to appear in rich results (star ratings, FAQ accordions, sitelinks) and are dramatically more likely to be cited by AI engines like ChatGPT, Perplexity, and Google AI Overviews because the data is unambiguous. Common types for service businesses: LocalBusiness, Service, FAQPage, Review, Article, BreadcrumbList, Person.
Example
A plumbing site adds LocalBusiness schema with serviceArea, openingHours, aggregateRating, and priceRange. Two weeks later it shows up in Google's local pack with star ratings.