What are the differences between testing functions and testing non-functional requirements?

Function testing (functional testing) aims to verify that the software performs its declared functions and meets the functional requirements. It checks how the system behaves with various input data and usage scenarios.

Non-functional testing verifies system characteristics not directly related to specific functions but related to the quality of operation: performance, reliability, security, usability, scalability, etc.

Main differences:

• Object of testing: functions and business logic vs. quality parameters.
• Methods: functional tests (unit, integration, system) vs. load testing, stress testing, security testing.
• Purpose: verify correct operation vs. verify quality and behavior under load or in special conditions.

Example: testing the "Save" button — functional testing; testing response time with 1000 concurrent users — non-functional testing.