Selenium is a popular open-source automation testing framework widely used for web application testing. It allows testers and developers to automate interactions with web browsers and perform various testing tasks such as functional testing, regression testing, and load testing. Selenium provides a suite of tools to support different aspects of automation testing, including:

1. Selenium WebDriver: It is the core component of Selenium and provides a programming interface to interact with web browsers programmatically. WebDriver allows testers to write test scripts in various programming languages such as Java, Python, C#, etc., and perform actions like clicking buttons, filling forms, navigating between pages, and validating page elements.

2. Selenium IDE: Selenium Integrated Development Environment (IDE) is a browser extension primarily used for record and playback of test scripts. It provides a simple graphical user interface that allows testers to record their interactions with the web application and later replay them to perform automated tests.

3. Selenium Grid: Selenium Grid is used for distributed test execution across multiple machines and browsers simultaneously. It allows running tests on various environments, operating systems, and browsers, thus enabling efficient cross-browser testing.

Advantages of using Selenium in automation testing:

1. Open-source: Selenium is freely available, which makes it cost-effective for organizations and individuals.

2. Cross-browser compatibility: Selenium supports various browsers like Chrome, Firefox, Safari, Edge, and Internet Explorer, making it suitable for cross-browser testing.

3. Multi-language support: Selenium WebDriver supports multiple programming languages, allowing testers to use the language of their choice for writing test scripts.

4. Extensive community support: Selenium has a vast user community, which means you can find ample documentation, tutorials, and community-driven tools to support your testing efforts.

5. Integration capabilities: Selenium can be easily integrated with popular testing frameworks like TestNG and JUnit for better test management and reporting.

6. Flexibility: It can be used for testing on various platforms like Windows, macOS, and Linux.

Limitations of using Selenium:

1. Limited support for desktop applications and mobile apps: Selenium is primarily designed for web applications, and its support for desktop applications and mobile apps is limited.

2. No built-in reporting: Selenium itself does not provide detailed reporting capabilities. Testers usually rely on integrating it with third-party frameworks for generating comprehensive test reports.

3. Synchronization issues: Selenium might face synchronization problems when dealing with dynamic web pages where elements load asynchronously.

4. Complex setups for Selenium Grid: Setting up and maintaining Selenium Grid for distributed testing can be challenging.

Overall, Selenium is a powerful and flexible tool for web application automation testing, and its popularity is mainly due to its versatility, community support, and cross-browser compatibility. It is important to choose the right test framework and practices to ensure a robust and maintainable test suite.