Selenium is an automation tool for the browsers. Selenium mainly used to automate the web applications. we can used to automate the desktop applications.
Selenium has the support of some of the largest browser vendors who have taken (or are taking) steps to make Selenium a native part of their browser.
It is also the core technology in countless other browser automation tools, APIs and frameworks.
Selenium is available in following flavors based on the use cases:
Selenium IDE :
Firefox add-on that will do simple record-and-playback of interactions with the browser.
I would not recommended to start learning selenium with IDE . However you can install add and check it how it records and runs the test.
Selenium RC ( Aka Selenium 1):
Selenium RC mainly works on client - Server architecture. It has been popular before driver comes into the picture. Most of the tests are written using selenium RC and maintained successfully.
Selenium Driver ( Aka Selenium 2):
S Selenium Driver is successor of the selenium RC . it does not uses any proxy server. it talks to api of the browser. it is a collection of language specific bindings to drive a browser -- the way it is meant to be driven
