What is the difference between functional and non-functional requirements?

Functional requirements describe what the system should do — specific functions and behaviors that ensure the achievement of business tasks. For example, "the user should be able to register" or "the system should send notifications".

Non-functional requirements define how the system should operate — quality characteristics such as performance, security, usability, scalability, and reliability. For example, "response time should not exceed 2 seconds" or "the system should provide data encryption".

Thus, functional requirements answer the question "what?", and non-functional requirements answer "how?".