What types of testing are considered non-functional and how do they differ from functional testing?

Non-functional tests verify not what the system does (functionality), but how it does it — its characteristics and quality. Main types of non-functional testing:

• Performance — testing speed, response time, throughput.
• Load testing — testing operation under expected load.
• Stress testing — testing behavior under extreme loads.
• Security — testing protection against attacks and vulnerabilities.
• Usability — assessing ease of use.
• Compatibility — testing operation on different devices, OS, browsers.

Unlike functional tests, which verify specific functions and business logic, non-functional tests evaluate the overall quality and characteristics of the system.