What is the difference between Selenium Standalone Server and Selenium Server?
The difference is that the Selenium Standalone Server is a local runner which offers a degree of speed, portability, and “cleanliness” that the Selenium Server lacks.
Selenium Standalone allows you to run a web driver directly from your machine. This offers several capabilities over a Selenium Server. Your tests will execute much faster, as there is no need to establish an HTTP connection to a remote service. You can also run multiple tests serially or in parallel, as there is no need for the control flow of a test harness such as Selenium Grid or Selenium RC. In the case where you have to use the Selenium Server due to restrictions in the browser configuration or other reasons, you can still take advantage of Selenium Standalone by using it as a synthetic remote browser.
The advantage of using Selenium Server is that it takes care of all the infrastructure details involved in managing test execution. You can set up a single server that manages all your tests, or use one per project, etc. The same goes for scaling and fault tolerance; these are handled automatically by the server.
To deep dive into the differences, please go through the following blog: