A unit test is a way to automate the diagnosis of your website. For example, let's say your website allows people to create a user profile. You could (A), verify that you can successfully create a profile by opening your web browser, navigating to your site, creating a profile, and making sure you get a successful response. That's fine and relatively quick to do. However, as your code base grows in size, you don't want to have to redo this test manually every time you add a new feature that could potentially break your user-profile feature. Thus, and (B), comes in unit testing (or integration tests, or simply tests), that automates the process for you. Every time you add a new feature, you can run tests to verify that this new feature didn't break something else in your code base.