https://stackoverflow.com/questions/30865539/selenium-server-standalone-jar-vs-selenium-java-jar
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
selenium1.0还是 seleniumRC的时候，需要启动selenium-server-standalone包，用来做server。
selenium RC通过server来给code和broswer建立通道，同时，该jar包包括我们用得所有的方法。
在新版的selenium中，即selenium2.0-webdriver，不需要这个selenium-server-standalone这个包了。
WebDriver api 会直接和浏览器的native交互，现在我们用selenium-java.jar包来替代。
https://stackoverflow.com/questions/26900631/what-are-the-differences-between-selenium-server-standalone-jar-and-selenium
Selenium WebDriver 2.0 - helps to write scripts for automating browsers..
this package provides us with classes & methods to achieve automation.
After writing scripts we can run them on LOCAL MACHINE and see automation ourselves.
WebDriver projects were merged with selenium RC to overcome the drawbacks of selenium RC making it selenium WebDriver 2.0 Selenium Server:
Now once i have my scripts (as mentioned above),
To run scripts on REMOTE MACHINES (Test Beds) and NOT ON LOCAL MACHINE we do it using selenium server.
So in short Selenium Webdriver works together with Selenium Server..
they co-exist to help and not to replace each other.
webdriver用于在本地执行，如果要远程自动化，就要+一个selenium server包