What is an apex Unit Testing? Why we need to write Test classes?

Hii sumit,
To facilitate the development of robust, error-free code, Apex supports the creation and execution of unit tests. Unit tests are class methods that verify whether a particular piece of code is working properly. Unit test methods take no arguments, commit no data to the database, and send no emails. Such methods are flagged with the @isTest annotation in the method definition. Unit test methods must be defined in test classes, that is, classes annotated with @isTest.