Functional testing and non-functional testing are two fundamental approaches used in software testing, each serving distinct purposes in ensuring the quality and reliability of software systems.

Functional Testing:

Functional testing primarily focuses on verifying that the software functions as expected according to its specifications. It ensures that the software meets the functional requirements outlined in the design and functional specification documents. This type of testing evaluates what the system does and whether it does what it's supposed to do.

Examples:

1. Unit Testing: This is a type of functional testing where individual units or components of the software are tested in isolation. For instance, in a banking application, a unit test might verify that the function responsible for transferring funds between accounts works correctly.

2. Integration Testing: Integration testing ensures that individual software modules work together as intended. For example, in an e-commerce platform, integration testing might validate that the product catalog module integrates seamlessly with the payment processing module.

3. System Testing: This involves testing the entire system as a whole to ensure that all components work together as expected. For instance, in a healthcare management system, system testing would verify that patient records are correctly created, updated, and retrieved across different modules.

Non-Functional Testing:

Non-functional testing evaluates the performance aspects of a system, such as its reliability, scalability, usability, and security. Unlike functional testing, which focuses on what the system does, non-functional testing assesses how well the system performs under various conditions.

Examples:

1. Performance Testing: This type of testing evaluates how the system performs under different load conditions. For instance, load testing determines how the system behaves when subjected to normal, peak, and overload conditions, ensuring it can handle the expected number of users without degradation in performance.

2. Usability Testing: Usability testing assesses the ease of use and user-friendliness of the software. For example, in a mobile banking application, usability testing might involve evaluating the intuitiveness of the user interface, the clarity of instructions, and the efficiency of completing common tasks like transferring funds or checking account balances.

3. Security Testing: Security testing identifies vulnerabilities and weaknesses in the software's security mechanisms. For instance, penetration testing involves simulating cyber-attacks to identify potential entry points for hackers and ensure that sensitive data is adequately protected.

4. Reliability Testing: Reliability testing assesses the stability and robustness of the software under normal and abnormal conditions. For example, in an online reservation system, reliability testing might involve verifying that the system can handle unexpected errors gracefully without crashing or losing data.

5. Scalability Testing: Scalability testing evaluates how well the system can handle increasing loads by adding resources such as users, transactions, or data volume. For instance, in a social media platform, scalability testing might determine how the system responds as the number of active users grows over time.

In summary, while functional testing focuses on verifying what the software does according to its specifications, non-functional testing assesses how well the software performs in terms of various quality attributes such as performance, usability, security, reliability, and scalability. Both types of testing are essential for ensuring the overall quality and reliability of software systems.