Schema.org is a collaborative project that defines a common set of terms for describing things on the web. It provides types (Article, Organization, Person, Product, FAQPage) and the properties each can have (name, author, datePublished, url).
It is a vocabulary, not a format. You choose how to write it down, and the most common choice for websites today is JSON-LD. Because the terms are shared, different search engines and tools can read the same markup.
Using it well means picking the most specific type that fits, filling in the properties that apply and ensuring the values match what visitors can see. Not every type leads to a visible feature in search, but valid markup still helps systems understand your page. Schema.org itself is not tied to any one search engine.
Related: Structured Data, Article Schema, Organization Schema, FAQPage Schema.