What are the different levels of testing?

Testing is conducted at different levels of the software development life cycle, each focusing on specific aspects of the application. The common levels of testing include:

Unit Testing:
- Scope: Individual units or components of the software, such as functions or methods.
- Objective: Verify that each unit functions correctly in isolation.
- Tools: Unit testing frameworks (e.g., JUnit, NUnit, PyTest).
Integration Testing:
- Scope: The interactions between integrated components or systems.
- Objective: Verify that integrated components work together as expected.
- Tools: Integration testing frameworks, testing frameworks with mocking support.
System Testing:
- Scope: The entire system as a whole, including end-to-end functionalities.
- Objective: Verify that the entire system meets specified requirements.
- Tools: Selenium, JUnit, TestNG, and other automated testing tools.
Acceptance Testing:
- Scope: Validating that the software meets acceptance criteria and is ready for release.
- Objective: Verify that the software satisfies user requirements.
- Types: User Acceptance Testing (UAT), Alpha Testing, Beta Testing.
- Tools: Behavior Driven Development (BDD) tools like Cucumber, JBehave.

These testing levels align with the traditional Waterfall model, where testing is conducted sequentially after each phase of development. In contrast, in Agile methodologies, testing is often performed concurrently with development, and there's a greater emphasis on continuous integration and continuous testing. In Agile, the testing levels are still relevant but may be performed more iteratively and collaboratively.

It's important to note that the distinction between these levels is not always rigid, and some testing activities may span multiple levels. For example, automated tests created during unit testing can contribute to regression testing during later stages.

Additionally, there are other types of testing that may not neatly fit into these traditional levels, including:

Regression Testing: Ensures that new changes do not adversely affect existing functionalities.
Performance Testing: Evaluates the performance, speed, and responsiveness of the software.
Security Testing: Identifies vulnerabilities and weaknesses in the software's security.

The testing levels and types chosen for a project depend on factors such as project requirements, development methodology, and the nature of the application. The goal is to have a comprehensive testing strategy that ensures the software's quality and reliability.