In the previous version of Selenium, which is Selenium RC (Remote Control), it is mandatory that you need to run selenium-server-standalone.jar
jar file, which acts like a server. Selenium RC will then use this server to establish a communication channel between browser and the code. Also, this jar file contains all the library functions to be used in our code.
But in later version of Selenium, which is Selenium WebDriver, there is no need to run this jar file, as the WebDriver api will directly communicate with the browser's native language. So, this jar file is replaced with selenium-java.jar
jar files
Hope this helps.