The Selenium Server which launches and kills browsers, and acts as an HTTP proxy for browser requests.

Client libraries for various programming languages, each of which instructs the Selenium Server in how to test the AUT bypassing it your test script’s Selenium commands.

The diagram shows the client libraries communicate with the Server passing each Selenium command for execution. Then the server passes the Selenium command to the browser using Selenium-Core JavaScript commands. The browser, using its JavaScript interpreter, executes the Selenium command, which effectively, runs the check you specified in your Selenese test script.