What is the difference between functional and non‑functional testing?

Functional testing verifies that the application behaves as expected from the user’s perspective, focusing on features, functions, and business rules. It checks inputs, outputs, and interactions. Non‑functional testing evaluates attributes that are not tied to specific functions, such as performance, scalability, usability, security, and reliability. While functional tests answer “Does it work?”, non‑functional tests answer “How well does it work?”. Both are essential: functional ensures correctness, non‑functional ensures quality attributes. For instance, a functional test might verify that a login page accepts valid credentials, whereas a non‑functional test might measure the page’s load time under 100 concurrent users. In practice, functional testing is often performed early in the development cycle, while non‑functional testing is conducted during performance testing, security audits, and usability studies. A balanced testing strategy includes both types to deliver a product that not only works but also meets user expectations for speed, reliability, and security.