Explain Selenium & Webdrivers Like I'm Five

Hey there!

I've been interested lately in applying end-to-end tests to my web application. I've seen several options available but I don't really understand the differences or how they work.

For instances, how does a remote selenium server work in practice? How does the remote selenium server have access to say a server running on my local machine (if thats how it works?)

This area is very new to me and am eager to learn more :)

Thanks!