WebDriver and Selenium Server

You may, or may not, need the Selenium Server, depending on how you intend to use Selenium-WebDriver. If you will be only using the WebDriver API you do not need the Selenium-Server. If your browser and tests will all run on the same machine, and your tests only use the WebDriver API, then you do not need to run the Selenium-Server; WebDriver will run the browser directly.
There are some reasons though to use the Selenium-Server with Selenium-WebDriver.

• You are using Selenium-Grid to distribute your tests over multiple machines or virtual machines (VMs).
• You want to connect to a remote machine that has a particular browser version that is not on your current machine.
• You are not using the Java bindings (i.e. Python, C#, or Ruby) and would like to use HtmlUnit Driver