Functional testing:

Functional testing is a crucial quality assurance process in software development that evaluates whether a software application functions correctly according to its specifications and requirements. It focuses on testing the functionality of the application, ensuring that it performs its intended tasks accurately.

Here is an example for functional testing

Consider a basic e-commerce website that allows users to browse products, add items to their carts, and complete a purchase. Functional testing for this application would involve testing various aspects of its functionality:

1. User Registration: verify that users can successfully create an account with valid information.

2. Product search: Test whether the search function accurately retrieves products based in keywords, categories or filters.

3. Add to cart: Check if users can add products to their cart and that the cart accurately displays the selected items and their quantities.

4. Checkout process: Validate that users can process through the checkout process smoothly. This includes entering shipping information, payment details and reviewing the order before confirming it.

5. Payment processing: Ensure that payment are processed accurately, with appropriate validation for credit card information, addresses and transaction confirmations.

6. Order confirmation: Verify that users receive an order confirmation email after completing a purchase.

7. User account: Test various account-related functionalities, such as updating personal information resetting passwords, and viewing order history.

8. Compatibility: Check if the website functions correctly across different browsers and devices.

Functional testing involves both positive testing and negative testing. Test cases are designed based on the applications functional requirements, and test results are compared against expected outcomes to identify any deviation or defects.

Automated testing tools and manual testing can be used for functional testing. Regression testing, which retests existing functionalities after changes, is also a vital part of the process to ensure that new updates or features do not break existing functionality

In summary, functional testing is essential for assuring the reliability and correctness of software applications, and it involves systematically testing each function and feature to ensure it meets the specified requirements like the example provided for e-commerce website

Non-functional testing:

Non-functional testing often referred to as quality testing, focuses on evaluating the characteristics of a software system that are not directly related to its functional behavior. These characteristics include performance, usability, reliability, scalability, security and more.

Performance testing assesses how well a software system performs under various conditions. It ensures that the application meets performance expectations and can handle a specified load.

Some example scenarios for performance testing:

1. Load testing: This assesses how the system performs under expected and peak loads. For instance, an e-commerce website, load testing would involve simulating a large number of concurrent users accessing the site to ensure it can handle heavy traffic during sales events.

2. Stress testing: Stress testing pushes the system beyond its specified limits. For example, lets take a mobile banking app, stress testing would involve initiating transaction with a higher-than-usual number of concurrent users to identify performance bottlenecks or system failures.

3. Scalability testing: Scalability testing checks if the system can handle increased load by adding more resources or servers. It ensures that the application can scale smoothly as the user base grows.

4. Response time testing: This measures how quickly the system responds to user actions. For a video streaming service, response time testing would evaluate the time it takes to start playing a video after a user clicks the play button

Usability testing:

It focuses on the user friendliness and overall user experience of the software. It ensures that the application is easy to navigate and meets user expectations. For example:

1. User interface testing: Evaluates the design and layout of the user interface. Testers assess whether buttons, menus and navigation are intuitive and user-friendly.

2. Accessibility testing: Checks if the application is accessible to users with disabilities, ensuring compliance with accessibility standards

Security testing:

Security testing identifies vulnerabilities and weaknesses in the software that could lead to breaches or data leaks

1. Penetration testing: Simulates cyberattacks to identifies vulnerabilities that hackers could exploit, suck as SQL injection or cross site scripting vulnerabilities.

2. Authentication testing: Verifies the effectiveness of users authentication mechanism, ensuring that only authorized users can access sensitive information.

So, non-functional testing is crucial to ensure that software not only functions correctly but also meets performance usability, security, and other critical criteria. Conducting these tests helps identify and mitigate risks and improve the overall quality of the software product.