Selenium IDE also known as the Selenium Recorder was made available for use in 2006. It offers an Integrated Development Environment to record and playback test scripts. It is available as a Mozilla Firefox add-on and a Google Chrome plugin.
The primary purpose of bringing in Selenium IDE was to ease the process of regression testing for manual testers. With Selenium IDE, all you have to do is hit the record button and run the test manually in your browser and once the test case is executed, you can stop the recording. The next time you need to execute that test case, you would just have to playback the recording and the browser automation would take place.
While Selenium IDE records your test cases, it automatically generates a Selenese script for every interaction that you make with the browser. Selenese is a scripting language exclusive to Selenium. It offers commands for various browser operations such as click a button, select values from drop-down etc.
Introduced in 2005, Selenium RC became the first instance of Selenium testing tool, which is why it is often referred to as Selenium 1. It was configured in Java to work on the basis of a client-server model where Selenium RC is the server & browsers were clients. As a server, Selenium RC receives commands from the browser client through HTTP protocol.
Although written in Java, Selenium RC allowed execution of browser automation tests written across any programming language. To further simplify the process, later Selenium Client API were introduced. Selenium Client API lifts the dependency of you having to write Selenese as they help Selenium to interpret the code written in different programming languages. You have Selenium Client drivers for Java, C#, Python, Ruby, PHP. For JavaScript, the same Java driver can be used through Rhino engine.
For parallel testing with Selenium RC, every new test case will need a different instance of server unless you are running parallel testing with Selenium RC through Java & PHP where the server instance runs in a continuous state.
