GraphQL is an API query language and runtime for executing those queries by providing a more efficient and flexible alternative to traditional REST APIs. It was developed by Facebook in 2012 and open-sourced in 2015. GraphQL has gained popularity in recent years because of its ability to address some of the limitations of RESTful APIs.

Here's an introduction to GraphQL:

1. Query Language: GraphQL is a query language used to request and manipulate data from a server. It allows clients to specify exactly what data they need, and nothing more. This contrasts with REST APIs, where endpoints typically return a fixed set of data, often leading to over-fetching or under-fetching of data.

2. Hierarchical Structure: GraphQL queries have a hierarchical structure, mirroring the shape of the response data. Clients can request nested fields, and the server returns data in the same shape. This eliminates over-fetching, as clients only receive the data they explicitly request.

3. Single Endpoint: Unlike REST, which often requires multiple endpoints for different resources, GraphQL typically exposes a single endpoint for all data queries and mutations. This simplifies API management and reduces the number of network requests.

4. Strongly Typed: GraphQL schemas are strongly typed, meaning they define the types of data that can be queried and their relationships. Clients can introspect the schema to discover available queries and types.

5. Resolvers: In GraphQL, servers define resolvers for each field in a type. These resolvers specify how to retrieve the requested data. This allows for fine-grained control over data access and enables integration with various data sources, such as databases, REST APIs, or third-party services.

6. Mutations: In addition to queries, GraphQL supports mutations, which are used to modify data on the server. Mutations are executed sequentially and can return results, just like queries.

7. Real-time Data: GraphQL can be used to subscribe to real-time updates through subscriptions. Subscriptions allow clients to receive data updates as they occur, making it suitable for applications requiring live data, such as chat apps or dashboards.

8. Validation and Type Checking: GraphQL clients can validate queries against the server's schema before sending them. This helps catch errors and reduces the chance of receiving unexpected data or errors in responses.

9. Versioning: GraphQL eliminates the need for versioning in APIs because clients request only the data they need. When adding new features or fields, changes can be made to the schema without breaking existing clients.

10. Tooling: GraphQL has a rich ecosystem of tools and libraries for various programming languages, making it easier to implement and work with.

GraphQL is a flexible and efficient API query language that empowers clients to request the exact data they need. Its hierarchical structure, strong typing, and ability to handle real-time data make it a popular choice for modern applications. It has been adopted by many companies and is commonly used in web and mobile app development.