ANSWER

Non-functional requirements are characteristics of a system or solution which describe non-behavioral characteristics or qualities of a system.

Whereas functional requirements describe the behaviors or functions of a system, the non-functional requirements generally describe those attributes which the system must have but which do not represent something an actor can do with the system.

Non Functional Requirements have also been called the 'ilities' because they can be expressed like this: usability, reliability, interoperability, scalability, extensibility, etc.  Non-functional requirements are also commonly referred to as quality of service (QoS) requirements or service-level requirements.

In many instances non-functional requirements describe (are attributes of) functional requirements. For example: if a functional requirement of a system is "the system shall allow the customer to view current month's statement" a non-functional requirement which describes this need might be "the system shall generate the current month's statement in 30 milliseconds" or "the system shall store monthly statements for up to 12 months".

In other instances, non-functional requirements describe constraints on a system such as legal and regulatory constraints.

A simple test to determine if a requirement is functional or non-functional is to ask yourself if you can describe the requirement using the action of an actor or user of a system (think use cases and user stories). If the requirement can be easily described with a use case or user story then it is probably a functional requirement, otherwise it is most likely a non-functional requirement.