What is a functional requirement?
Refers mainly to the expected function of the product. Product or system features, or what the program does is what the functional requirements are. Definitely it is easier to determine the functional requirements than the non-functional because they are almost obvious.
What is a non-functional requirement?
These requirements refer mainly to the attributes that include performance, security, usability, etc. Developers usually miss these requirements because they focus mainly on the functional requirements. When addressing incorrectly these non-functional requirements, the system actually fails. For example, the lack of usability or performance can lead to frustration or system problems. This is why non-functional requirements are so important for a system to succeed.
