GraphQL is a query language for application programming interfaces (APIs), and a runtime for fulfilling data requests.
As GraphQL calls are handled in a single round-trip, clients only get what they request and do not download excessive amounts of data. Thanks to that, the applications using this runtime can be fast even on slow internet connections.
GraphQL APIs are organized by types and fields, not endpoints. Clients can define the structure of the required data, and the same structure of data is returned from the server. This prevents miscommunication between client and server. From the end-user perspective, the use of types ensures that applications only request what is possible and give clear errors, contributing to a better user experience.
As another advantage GraphQL allows evolving of an API without versions. Adding new fields and types is possible without impacting the existing queries. This supports cleaner and better maintainable code.
The main difficulties of GraphQL are related to its differences from the widespread alternative solution – the REST APIs. First of all, it introduces a learning curve for developers who are familiar with REST. Using GraphQL, caching is more complex than with REST. Also, it shifts much of the work to the server side, adding complexity there. Moreover, different API management strategies can be necessary.
It depends on the development approach as well as business goals whether the value outweighs the complexities. For simple APIs, it might not be advantageous, but it can be a great solution for aggregating data from multiple places into one API, adding functionalities to old APIs, simplifying complex APIs, and etc.
Read more about GraphQL on their website.