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

What is the difference between functional and non-functional requirements in the context of designing a software system?

Give examples for each case.

A functional requirement describes what a software system should do, while non-functional requirements place constraints on how the system will do so.

Let me elaborate.

An example of a functional requirement would be:

• A system must send an email whenever a certain condition is met (e.g. an order is placed, a customer signs up, etc).

A related non-functional requirement for the system may be:

• Emails should be sent with a latency of no greater than 12 hours from such an activity.

The functional requirement is describing the behavior of the system as it relates to the system's functionality. The non-functional requirement elaborates a performance characteristic of the system.

Typically non-functional requirements fall into areas such as:

• Accessibility
• Capacity, current and forecast
• Compliance
• Documentation
• Disaster recovery
• Efficiency
• Effectiveness
• Extensibility
• Fault tolerance
• Interoperability
• Maintainability
• Privacy
• Portability
• Quality
• Reliability
• Resilience
• Response time
• Robustness
• Scalability
• Security
• Stability
• Supportability
• Testability

A more complete list is available at Wikipedia's entry for non-functional requirements.

Non-functional requirements are sometimes defined in terms of metrics (i.e. something that can be measured about the system) to make them more tangible. Non-functional requirements may also describe aspects of the system that don't relate to its execution, but rather to its evolution over time (e.g. maintainability, extensibility, documentation, etc.).