1. Unit Testing:
Unit testing is the foundation of software testing and involves testing individual components or units of code in isolation. Developers typically perform unit tests to validate that each unit functions as intended and produces the expected output. By isolating units of code, developers can easily identify and fix defects early in the development process, resulting in more robust and maintainable codebases. Unit testing frameworks such as JUnit, NUnit, and XCTest provide developers with tools to automate and streamline the unit testing process, making it an integral part of modern software development practices like Test-Driven Development (TDD) and Behavior-Driven Development (BDD).

2. Integration Testing:
Integration testing focuses on verifying the interactions between different units or modules within a software system. Unlike unit testing, which tests individual components in isolation, integration testing examines how these components work together as a whole. Integration tests help uncover issues related to data flow, communication between modules, and compatibility between different parts of the system. By validating the integration points early in the development lifecycle, teams can mitigate the risk of integration failures during later stages of development or deployment. Techniques such as top-down integration testing, bottom-up integration testing, and sandwich integration testing are commonly used to execute integration tests effectively.

3. System Testing:
System testing evaluates the entire software system as a whole and validates its behavior against specified requirements. It focuses on assessing the system’s functionality, performance, security, and reliability under various operating conditions. System tests are typically black-box tests, meaning they are conducted without knowledge of the internal workings of the system. Common types of system tests include functional testing, usability testing, performance testing, security testing, and compatibility testing. System testing helps identify defects that may arise due to interactions between different components or subsystems, providing stakeholders with confidence in the overall quality of the software product.

4. Acceptance Testing:
Acceptance testing, also known as user acceptance testing (UAT), is the final phase of testing before software is released to end-users. It involves validating that the software meets the requirements and expectations of its intended users or stakeholders. Acceptance tests are often conducted by end-users or product owners in a real-world environment to simulate how the software will be used in production. The primary goal of acceptance testing is to ensure that the software delivers the intended value and functionality while meeting quality standards and regulatory requirements. Successful completion of acceptance testing signals readiness for deployment and signifies that the software is fit for release.