Non-functional testing is a critical aspect of software quality assurance that focuses on evaluating aspects of a system beyond its functional requirements. Unlike functional testing, which assesses the specific behaviors and functionalities of an application, non-functional testing examines criteria such as performance, usability, reliability, security, and scalability.

Key types of non-functional testing include:

1. Performance Testing: This evaluates how the system performs under various loads. It includes load testing, stress testing, and endurance testing to ensure that the application can handle the expected number of users and data transactions without degradation in performance.

2. Usability Testing: This assesses how user-friendly and intuitive the application is. It involves real users interacting with the system to identify areas for improvement in design, navigation, and overall experience.

3. Security Testing: This ensures that the application is free from vulnerabilities and protects sensitive data from unauthorized access. Techniques such as penetration testing and risk assessment are used to identify and mitigate potential security threats.

4. Compatibility Testing: This checks the application's performance across different devices, operating systems, web browsers, and network environments to ensure consistent functionality and appearance.

5. Reliability Testing: This focuses on the reliability and stability of the application over time. It assesses the system's ability to perform consistently under expected conditions and to recover from failures.

6. Scalability Testing: This evaluates how well the application can scale up or down in response to varying loads. It tests the system's capacity to handle increasing numbers of users or transactions without compromising performance.

By prioritizing non-functional testing, organizations can enhance user satisfaction, ensure compliance with regulatory requirements, and ultimately improve the overall quality and resilience of their software products. This holistic approach to testing contributes to the successful deployment and long-term viability of applications in a competitive marketplace.