What is the difference between functional and non-functional requirements? What non-functional requirements do you know?

Functional requirements describe what the system should do — the specific functions and behaviors that the application implements. For example, "the user can register," "the system sends notifications."

Non-functional requirements define how the system should operate, i.e., qualities and constraints not directly related to functionality. They affect performance, reliability, usability, and other characteristics.

Examples of non-functional requirements:

• Performance (response time, throughput)
• Reliability and fault tolerance
• Security (encryption, authentication)
• Scalability
• Usability
• Compatibility with other systems
• Localization and internationalization
• Maintainability and extensibility

Thus, functional requirements answer the question "what to do?", and non-functional requirements answer "how well to do it?".