As mentioned before, GraphQL is a query language that lets you define what the response of an API looks like by how you structure your query. Not only is it a query language, but it also provides a runtime to fulfill those queries based on your existing data. Not only can you use GraphQL to fetch data with queries, but you can also send mutate data by using mutations.
When you want to use GraphQL, the API that you're using for data fetching should support GraphQL. This means the server should have a schema that describes which operations (queries, mutations, or subscriptions) are allowed and which data fields can be requested. Every operation that is described in the schema for a GraphQL server can be executed by sending a document containing these operations. Other than with REST APIs, you have complete control over the shape of your data as you define what structure the response should have in your operation.
Let's get a taste of how GraphQL queries...