Source: 📖 Test Driven Development with Python ch2 p13
Functional tests are designed to let us test how the application functions from the user's perspective. To achieve this in web development, we can use Selenium as with it we can drive a real web browser. This means that we can use functional tests as a kind of specification for our app—we can define how our app should behave by establishing test criteria.
Functional tests, acceptance tests, end-to-end tests and black box tests all refer to the same thing. Essentially, they are about testing how an app functions from the outside. They are not concerned with the inner workings of the program, they only test the user experience.