Talking about TestNG, it is an open source testing framework. It was inspired by JUnit.
TestNG stands for Test Next Generation.
It is more powerful than Selenium webdriver and JUnit and easier to use.
It has more functionalities than JUnit.
This was What is TestNG.
Talking about Why TestNG.
As we know that Selenium WebDriver cannot generate proper reports, but when it is combined with TestNG, you can generate reports of proper format and it also overcomes the limitations of JUnit.