How do I organize structure of my testSuite using beforeClass, afterClass, afterSuite in testNG?

I m confused about using annotations @beforeClass,afterClass, beforeSuite,afterSuite in testNG.

I understand test structure below:

package mytestNG.learning.it ;

public class sample_not_working {
  @Test
  //take action - click on links, input data etc
  public void main() {
  }
  @BeforeMethod
  //do stuff like setup browser etc
  public void beforeMethod() {
  }

  @AfterMethod
  //close browser 
  public void afterMethod() {
  }
}

But what do you do in beforeclass,afterClass and test? What file is it? Is it a class that runs other classes?

Next, afterSuite, beforeSuite and @test :

  public class sample_not_working {
  @Test

  public void main() {
  //WHAT KINDA CODE YOU PUT HERE?
  }
  @BeforeSuite
  //WHAT KINDA CODE YOU PUT HERE?
  public void beforeMethod() {
  }
  @AfterSuite
  public void afterMethod() {
  //WHAT KINDA CODE YOU PUT HERE?
  }
}

My question is about semantics, the meaning, not actual code. I read the testNG docs - did not help.