How does 'functional testing' differ from 'non-functional testing'?

Functional testing verifies that specific functions of the software operate according to the requirements and design specifications. It involves evaluating features such as user interactions, business processes, and input/output processing. The primary goal is to ensure that the application performs its intended tasks correctly and meets the specified requirements.

Non-functional testing, on the other hand, assesses aspects that are not related to specific functionalities. This includes performance testing, which measures how the system behaves under load; usability testing, which evaluates the user experience and ease of use; and other criteria such as security, reliability, and scalability. Non-functional testing is crucial for determining how well the application performs in real-world scenarios, beyond just its functional capabilities.

This distinction underlines that while functional testing is concerned with "what" the system does, non-functional testing is concerned with "how well" it performs those functions.