Creating a WDS Network

I have done an extensive search of the forums, and a lot has been written about this subject, but none of it seems to answer my question (either none of it actually answers the question, or I am just dumb). Here is my situation. I have one Airport Extreme Base Station that is hooked in via ethernet to my Comcast cable modem. Also, I have one Airport Express that is hooked in to my surround sound system across the house. So far, no WDS system has been implemented. What I would like to do is continue to use my AEBS where it is, continue to stream music to the Express, and add another express to the second floor to boost the signal up there.
In order to accomplish this, I assume I need to create a WDS network. So, I would make the AEBS the Main, the Music Express a remote, and the new, third one a remote? Also, what is the difference between a remote and a relay?

Thanks

 

For argument's sake, could I set the AEBS as the main, then the music express as a relay, and the new one as a relay.
Also, what does a remote actually do?