GraphQL is a query language for APIs and a runtime for executing those queries against a type system that you define for your data.

Unlike a traditional REST API, where the server exposes multiple endpoints such as /users, /users/42, and /users/42/posts, GraphQL generally exposes a single endpoint through which the client describes the data it needs.

The important idea is that the client does not simply ask the server for a resource. It asks for a specific shape of data.

For example, a client might request:

query {
  user(id: "42") {
    id
    name
    email
  }
}

The response follows the structure requested by the client:

{
  "data": {
    "user": {
      "id": "42",
      "name": "Suriya",
      "email": "suriya@example.com"
    }
  }
}

GraphQL itself does not require MongoDB, Node.js, React, Apollo, or any particular programming language. It defines a specification for describing an API schema, querying that schema, validating operations, and executing them.

A GraphQL server typically has three important responsibilities:

1. Define the schema that describes available data and operations.
2. Validate incoming operations against that schema.
3. Execute valid operations using resolver functions.

This separation between the API contract and the underlying data sources is one of GraphQL's most important architectural characteristics.