GraphQL is a query language for APIs (Application Programming Interfaces) and a runtime for executing those queries with your existing data. It was developed by Facebook in 2012 and later open-sourced in 2015.

Here are some key features and concepts associated with GraphQL:

1. Declarative Data Fetching: Clients can specify the structure of the response they need, and the server returns only that data. This helps in reducing over-fetching or under-fetching of data, as clients get exactly what they request.

2. Hierarchical Structure: GraphQL queries have a hierarchical structure, mirroring the shape of the data that will be returned.

3. Strong Typing System: GraphQL APIs are strongly typed, and the types are defined by the server. Clients can query for specific fields and types, ensuring a predictable response.

4. Single Endpoint: Unlike traditional REST APIs that may have multiple endpoints for different resources, GraphQL typically exposes a single endpoint for all interactions. This simplifies the API surface.

5. Real-time Data with Subscriptions: GraphQL supports real-time data updates using subscriptions. Clients can subscribe to specific events and receive updates when those events occur.

6. Introspection: GraphQL APIs are introspective, meaning that clients can query the schema itself to discover the types, fields, and operations supported by the server.

7. Mutations: In addition to queries for reading data, GraphQL has a concept of mutations for making changes to the data (e.g., creating, updating, or deleting).

8. Resolvers: Resolvers are functions responsible for fetching the data for a particular field in the schema. They allow developers to specify how data should be retrieved or manipulated.

9. Tooling: GraphQL has a rich ecosystem of tools, libraries, and extensions for various programming languages. This includes client libraries for making GraphQL requests, server frameworks, and tools for schema validation and exploration.

10. Schema: The schema is a central concept in GraphQL. It defines the types of data that can be queried and manipulated, along with the relationships between them.

GraphQL has gained popularity for its flexibility, efficiency, and the ability to empower clients to request exactly the data they need. It's commonly used in web and mobile app development, and many companies have adopted it as an alternative or complement to RESTful APIs.