GraphQL and REST are two prominent API styles, each with distinct methods for client-server communication. GraphQL, developed by Facebook, allows clients to specify precisely the data they need from a single endpoint, thus minimizing over-fetching and under-fetching issues, while REST uses multiple resource-based endpoints with fixed payloads, aligning well with HTTP methods and caching. GraphQL's flexibility in querying and schema evolution makes it suitable for complex, evolving applications, while REST's straightforward implementation, strong caching capabilities, and mature tooling make it ideal for simpler applications and public APIs. Both approaches have unique strengths; GraphQL excels in scenarios where dynamic data fetching is needed, whereas REST is favored for its simplicity and efficiency in predictable, resource-based APIs. API testing remains crucial for both, ensuring reliable data exchange and performance across different use cases.