Question

Define and differentiate functional and non functional requirements.

Answer 1

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.).