Functional Testing:

Functional testing verifies the system against the functional requirements and specifications. Functional testing verifies each function of the software/system to meet its functional requirements. It test the features and functionalities of the software, such as input/output, error handling, and user interface. Each functionality of the software application is tested by providing appropriate test input, expecting the output and comparing the actual output with the expected output.
Functional testing test Mainline functions, Basic Usability, Basic Accessibility, Error Conditions.

For example, if we take Amazon application we would check whether it can perform basic operations like adding the items to cart, checkout, adding address, payment etc.. correctly or not. Also, we would check if the user interface is user-friendly, like is it easy to search for the item, whether categories button works properly or not etc..

Few more verification that functional testing perform are:

• Whether users can easily log into the application or not with the correct credentials?
• When a user clicks “Buy Now”, does the UI take them directly to the next required page?
• If a user enters special characters in a field where they are not allowed, does the system throw up an error message and refuse to proceed?

There are many types of functional testing:

• Unit testing
• Integration testing
• User acceptance testing
• Closed-box testing

Non-Functional testing:

Non-functional testing is performed to verify the non-functional requirements of the application. It test the readiness of a system as per non-functional parameters. Non-functional testing assesses application properties that contribute to the end-user experience, like performance, stability, security, efficiency, usability, reliability, and compatibility. A fault in these will break the user experience. It make sure that the application meets the quality standards and performs well under different conditions/loads. It reduces the risk of bugs emerging in production when software is actually used by end-users. And simplifies the mechanisms related to installation, configuration, management and monitoring the software.

There are some special automated tools are used for Non-functional testing like LoadRunner, JMeter etc.

For example, In Amazon app there are millions of user handling the application at the same time, so it would check whether it can handle many concurrent users without slowing down or crashing. Also, it checks if the application is secure from potential cyber threats like hackers or viruses. And it checks the accessibility of the applications. Also, Amazon can be used in various languages now, so as a Non-functional tester we should make sure it is easy for people to use the app in their comfortable language.

There are many types of non-functional tests:

• Performance testing
• Load testing
• Usability testing
• Security testing

Functional and Non-Functional testing Comparison:

• Functional testing verifies the operations and actions of an application while the Non-Functional testing verifies the behaviour of an application.
• Functional testing can be done manually whereas Non Functional testing is hard to perform manually.
• Functional describes what the product does whereas Non Functional describes how the product works.
• Functional testing tests whether the actual result is working according to the expected result whereas Non-Function testing checks the response time, and speed of the software under specific conditions.