Selenium is a free (open-source) automated testing framework used to validate web applications across multiple browsers and platforms. Selenium Test Scripts can be written in a number of programming languages such as Java, C#, and Python. Selenium Testing refers to testing carried out with the Selenium testing tool. 

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The Selenium Integrated Development Environment (IDE) is the most basic and easiest to learn framework in the Selenium suite. It is a Firefox plugin that you can install just like any other plugin. Selenium IDE, on the other hand, should only be used as a prototyping tool due to its simplicity. If you want to create more advanced test cases, you must use Selenium RC or WebDriver.

In many ways, the WebDriver outperforms both the Selenium IDE and the Selenium RC. It uses a more modern and stable approach to automate browser actions. Unlike Selenium RC, WebDriver does not rely on JavaScript for Selenium Automation Testing. It commands the browser by communicating with it directly.

The Selenium Software Testing Suite is made up of four parts:

• Selenium IDE is a Firefox add-on that can only be used to create simple test cases and test suites.
• Selenium Remote Control, also known as Selenium 1, was the first Selenium tool that allowed users to create complex tests using programming languages.
• WebDriver is a newer breakthrough that allows your test scripts to communicate directly with the browser, allowing you to control it from the operating system level.
• Selenium Grid is a tool that works in tandem with Selenium RC to run parallel tests across multiple browsers and operating systems.
• Selenium 2 was created by combining Selenium RC and WebDriver.
• In terms of cost and flexibility, selenium outperforms QTP. 

Selenium 2:

Selenium2 is nothing more than the combination of WebDriver and Selenium RC (Selenium1). Selenium 1 is a well-known framework that, thanks to its JavaScript implementation, supports a wide range of browsers. To exit the JavaScript Sandbox, WebDriver is developed for each browser, providing a headless browser emulator that is very fast. Selenium2 incorporates the strengths of both WebDriver and Selenium 1, as well as their respective drawbacks.

• Selenium 2 is a combination of WebdDriver and SeleniumRC (Selenium1), both of which are based on the selenium core.
• WebDriver contributes an object-oriented API for interfacing with the Document Object Model (DOM) and controlling the browser. The default browser is Firefox.
• Supports the Selenium RC API as well as the Selenium RC technology hidden beneath the WebDriver API.
• Mozilla, Chrome, and Internet Explorer all include a Selenium-developed driver.
• For mobile automation, use AndroidDriver and iPhoneDriver.

Selenium 3:

This is a drop-in replacement for WebDriver API users. The main difference is that the core has been removed and replaced by the back-end WebDriver. W3C (World Wide Web Consortium) has approved Selenium 3.0 as a standard. Furthermore, Selenium 3 is primarily intended to be a software testing tool for both web and mobile-based applications.

• The core has been removed from Selenium3, but it still supports Selenium RC indirectly via the back-end Webdriver.
• Browser vendors will ship their own WebDriver implementation, which will be tightly coupled to the browser and thus provide a better testing experience.
• There is no Web Driver API. GeckoDriver has been added to Firefox (v47 and up), regardless of Selenium version.
• WebDrivers are introduced with modern browsers such as Edge and Safari by their vendors.
• For mobile automation, use tools like Appium.