REST Assured is a Java library (a DSL) for testing REST APIs. It provides a readable, given/when/then syntax to build HTTP requests, send them, and validate responses without writing low-level HTTP boilerplate.
Built on top of HTTP clients, REST Assured lets Java/SDET teams write API tests that read almost like English. It handles request building (headers, params, body, auth), sends the call, and offers powerful response validation via JsonPath/GPath, Hamcrest matchers, and JSON-schema checks. It serializes Java objects (POJOs) to JSON and deserializes responses back, integrates with TestNG/JUnit for structure and reporting, and pairs with Maven/Gradle. Compared to hand-rolling HttpClient code, REST Assured is far more concise and expressive, which is why it's the de facto Java API-testing tool and a frequent interview topic.
Instead of ~30 lines of Apache HttpClient code to GET a user and parse JSON, REST Assured does it in one readable statement: given().when().get('/users/1').then().statusCode(200).body('name', equalTo('QA')).
In an interview: 'Why use REST Assured instead of writing HTTP calls with Java's HttpClient?'