Selenium Grid extends Selenium RC by running tests on different servers in parallel or on multiple instance of browsers on the same server. It reduces the time, and subsequently the cost, for testing in various browsers under multiple operation systems.
Selenium Grid has two components: Selenium Hub and Remote Control. The hub receives requests from tests (the same as Selenium RC), and then allocates remote controls that are registered to the hub to each tests as shown on the picture below.
If QA engineers are familiar with Selenium RC, they will be able to easily run tests with Selenium Grid. QA engineers can also run existing code in parallel on multiple machines, which saves test execution time and quickly yields feedback of test results. However, Selenium Grid itself does not provide a parallel running strategy, so If QA engineers want to take advantage of Selenium Grid, they need to write your selenium tests in parallel mode.