Testng Class and a normal Java class
TestNG Class: TestNg class is nothing but any normal class with TestNG annotation and Annotation is a tag that represents the metadata which gives the addition information about class, methods and interface.
To know more about annotation Please refer this https://docs.oracle.com/javase/tutorial/java/annotations/
While creating Automation suite we use either JUNIT or TestNG. JUNIT or TestNG are testing framework. They have their own set of libraries and annotations which is useful to run our automation suite. JUNIT or TestNG makes easy to run automation suite or generate reports of build run. They provide the feature to setup or cleanup activity using Before and After annotation which is require in almost every autoamtion suite.
TestNG also provide us flexibility to execute our automation script based on our requirement like we can group of test cases, we can include or exclude methods, parallel execution of test methods and many more.