Unit Testing isolates each part of your application and checks that each individual part is correct. It ensures that any existing problems are discovered early on in the development cycle.
Catching bugs and software issues early ensures they are not given a chance to progress and embed deeper into your program. This saves you higher testing and developmental investments in the future, of both time and money.
In Unit Testing, each software module, together with the associated control data, usage procedures and operating procedures, are tested to determine whether they are fit for use. We also use substitutes such as method stubs, mock objects, and test harnesses to assist in testing a module in isolation.
Unit Testing is then followed by Regression Testing after any bugs have been fixed. And then Integration Testing, after the successful completion of Unit Testing.