What is functional and non-functional testing?

Functional testing verifies whether the system's behavior meets its functional requirements. It focuses on what the system should do — testing functions, business logic, user scenarios. For example, verifying that a form is successfully submitted when correct data is entered.

Non-functional testing evaluates characteristics of the system not directly related to its functionality, such as performance, security, usability, and reliability. For example, testing how many users can simultaneously use the application without a decrease in speed.

Thus, functional testing answers the question "What does the system do?", while non-functional testing answers "How does the system do it?".