In software testing, two critical categories of tests exist.

• Functional testing
• Non-functional testing

Functional Testing
Functional testing primarily focuses on verifying that the software operates according to the specified functional requirements. This type of testing checks the user interface, APIs, databases, security, and other essential functionalities by comparing the actual output to the expected results.

For example, consider an e-commerce website.
Functional testing in this case would verify that users can successfully search for products, add items to their cart, complete the checkout process, and receive order confirmation.
Test cases would be created for all these features, and testers would execute them to ensure that the system behaves as per the functional requirements.

Non-Functional Testing
Non-functional testing is concerned with the performance, usability, reliability, and overall quality attributes of the software. It tests the "how" rather than the "what" of the system.
Non-functional testing evaluates characteristics that may not be related to specific functionalities but are crucial for the software to perform well under various conditions.

For example, consider an e-commerce website.
Non-functional testing would assess how quickly the website loads under normal traffic conditions, how it handles a surge in visitors during sales events, and how secure the payment processed from cyber threats.
Performance testing, load testing, stress testing, security testing, and scalability testing are all types of non-functional tests that ensure the system performs adequately and remains stable under varying loads and conditions.

Functional testing ensures that the software's features work as intended, whereas Non-functional testing ensures that the system's performance and quality meet the necessary standards under varying conditions.

Both are crucial in delivering reliable, user-friendly, and high-performing software. By integrating both testing methods into the development lifecycle, organizations can ensure their software not only functions properly but also performs well in the real world.