Functional vs Non-functional Requirements

Feature	Functional Requirements	Non-functional Requirements
Purpose	Helps to understand the functions of the system	Helps to understand the system's performance
Mandatory	Mandatory	Not Mandatory
Ease of Definition	Easy to Define	Hard to Define
Description	Describes what the product does	Describes the working of the product
Focus	Concentrates on the user's requirement	Concentrates on the expectation and experience of the user
Verification	Helps to verify the software's functionality	Helps to verify the software's performance
Specification	Specified by the user	Specified by software developers, architects, and technical persons
Testing	Functional testing (API testing, system, integration, etc.)	Non-functional testing (usability, performance, stress, security, etc.)
Examples	Authentication of a user on trying to log in to the system	The background color of the screens should be light blue
Importance	Important to system operation	Not always important, may be desirable
System Operation	Completion of Functional requirements allows the system to perform	System will not work only with non-functional requirements