What is an Apex Test Class and why is it required?

An Apex Test Class is Apex code marked with @isTest that exercises your production Apex classes. Salesforce requires:

• 75% line coverage for the org's Apex code before deploying to production.
• Every trigger must be touched by at least one test.
• Tests must run successfully — no failures allowed during deploy validation.

Anatomy:

@isTest
private class AccountTriggerTest {
    @isTest
    static void testInsert() {
        Account acc = new Account(Name='Test');
        Test.startTest();
        insert acc;
        Test.stopTest();
        System.assertEquals('Test', [SELECT Name FROM Account WHERE Id=:acc.Id].Name);
    }
}

Key features:

• Test isolation — tests don't see existing org data by default. You build test data with @testSetup methods or inline factories.
• `Test.startTest()` / `Test.stopTest()` — async ops fired between these run synchronously when stopTest() is called, letting you assert on their effects.
• `System.assertEquals` — assertions that fail the test if values don't match.

Don't write tests just to hit coverage. Write tests that assert business behaviour — coverage is a side effect of good tests.