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

Functional requirements describe what the system should do — specific functions and behaviors that need to be implemented. For example, "the user should be able to register," "the system should send notifications."

Non-functional requirements describe how the system should operate, i.e., characteristics and constraints not directly related to functionality. For example, performance, security, usability, scalability, reliability.

Example:

• Functional requirement: "The system should allow the user to upload files up to 10 MB in size."
• Non-functional requirement: "The system's response time should not exceed 2 seconds when uploading a file."