Defining requirements can be difficult because there needs to be a distinction between functional and non-functional requirements; however, both of these requirements are important to the system, and without them the system can crash or fail. The problem is when people miss these non-functional requirements, either they know about it or not.
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.
Example
Both of them are needed specifications.
References:
