On Selenium download page, there is a link to the Selenium RC (Remote Control).
On another Selenium download page, there are links for a Selenium RC 1.0.3>
The documentation on Selenium's website says that the Remote Control consist of
The server is responsible for doing the actual work (executing actions in the browser) while your specifications for actions (the control of execution) comes from the different libraries.
This is called the Remote Control because the server does not have to be deployed on the same machine where the actual scripts are executed. The scripts can connect to a remote selenium server over the network and execute actions.
To sum up, when you are using the Selenium Remote Control, you are using the server + client libraries. They are meant to be used together.
I do not know if this is going to be the same for Selenium 2 or if they are changing things. I am only confident this is how things work for Selenium 1.