What Is JSON Schema? JSON Schema is a grammar language for defining the structure, content, and (to some extent) semantics of JSON objects. To define what JSON Schema is, we should probably first define what JSON is. JSON Schema is a powerful tool for validating the structure of JSON data. JSON Schema is a vocabulary that allows you to annotate and validate JSON documents. JSON - Schema - JSON Schema is a specification for JSON based format for defining the structure of JSON data. The what and why of JSON(Schema).