Selenium automates browsers.Selenium is an Open Source(Freely Available)and combination of software tools which is used in automation testing of web applications across different browsers and platforms.
Selenium is not a single tool, instead it is a set of different tools each with a different approach to support test automation. Learning all the tools will give you different options for approaching different test automation problems.These tools when used together give many options for locating the UI elements and comparing expected test results against actual application behavior. The following are the four tools that comprise the Selenium's Tools Suite:
It consists of 4 components:-
Selenium IDE ( Integrated Development Environment)
Selenium RC( Remote Control)
Selenium IDE is a Firefox plugin which allows users to record and play back actions in the browser.it can also play the test back and save them as a program in different languages
Selenium-RC was the first tool in the Selenium project that allowed automation of web applications in browsers. This has been deprecated, although it is still functional in the project, and WebDriver is now the recommended tool for browser automation.
Selenium WebDriver is the Selenium project supported browser automation tool. It consists of a set of libraries for different programming languages and ‘drivers’ (small applications) which can automate actions in browsers.
Selenium Grid allows you to run automation remotely on multiple browsers, and on other machines.in Other words it enables distributed test Execution.