TestNG is an open source automated testing framework inspired From JUnit and NUnit. The NG in TestNG denotes Next Generation; as you may see the name itself is self-explanatory indicating more functionalities which are added to meet today’s testing needs so as to deliver agility and better user experience. It is similar to JUnit but designed to be better than it. Cedric Beust is the creator of TestNG.
There are numerous benefits in TestNG but from Selenium point of view, we can quickly produce HTML Reports of execution, enjoy clear annotations, grouped & prioritize test cases more easily, can try parallel testing, generate logs and while also can take up data parameterization.
A best example of using TestNG framework in automation testing is discussed here https://goo.gl/sfkw6E