Tell about the differences between functional and non-functional testing.

Functional testing verifies that the system does what it is supposed to do (functions) according to requirements. Focus on "what?". Non-functional testing checks how well the system performs its functions, as well as other aspects not directly related to functionality (performance, security, reliability, usability, etc.). Focus on "how?".

Aspect	Functional Testing	Non-Functional Testing
Objective	Verify compliance with functional requirements	Verify system quality and performance
Test objects	Functionality, logic, business processes	Performance, security, usability, etc.
Types of testing	Unit, integration, system, acceptance	Load, stress, security, usability
Requirements	Functional requirements (User Stories, Use Cases)	Non-functional requirements (SLA, standards)
Focus	Checking functions, inputs, outputs, and their compliance	Checking system characteristics and behavior
Sample questions	Does the button work? Are data saved correctly?	How fast does the page load? Is the system load-resistant?
When conducted	At all stages of development	Usually after stabilization of functionality