Usages of TestNG Annotations on the Automation Framework

Before explaining the difference, first this is some testing terminologies

Test suite – Consists of one or more test tags.

Test tag - Consists of one or more test classes.

Test class – Consists of one or more methods.

for examble:

<suite name="suit1">
  <test name="TestTag1">
    <classes>
      <class name="TestClass1"/>
    </classes>
  </test>
  <test name="TestTag2">
    <classes>
      <class name="TestClass2"/>
      <class name="TestClass3"/>
    </classes>
  </test>
</suite>

@BeforeTest : It will be called Only one time before any test tag, no matter how many test classes inside that tag or how many method annotated with @Test, it will be called only one time for each test tag,in the previous XML example @BeforeTest will be called twice, one time beforeTestTag1 the second time before TestTag2 so it can be used to initialize common objects between different test classes inside one test tag.

@BeforeClass : It will be called Only one time before any test class, no matter how many method annotated with @Test inside this test class, it will be called only one time for each test class,in the previous XML example @BeforeClass will be called three times, one time beforeTestClass1 the second time before TestClass2 and the third time before TestClass3 so it can be used to initialize common objects between different test methods inside one test class.

@BeforeSuite will be called once for the suit1 suit

the order of calls will be as follow

@BeforeSuite
    @BeforeTest
        @BeforeClass
            @BeforeMethod
                @Test



Ref: 
https://stackoverflow.com/questions/30587454/difference-between-beforeclass-and-beforetest-in-testng