Please explain Babylon 5 to us!

We at io9 know about a lot of science fiction and fantasy stuff, but we have some gaps in our knowledge. So sometimes, we ask you, the readers, to help plug those gaps and enlighten us about your favorite stuff. Today, we're asking you to educate us about Babylon 5, the space opera TV show that aired from 1994 to 1998.

The questions in comments below range from the basic to the more complicated. And yes, we're asking for major spoilers ā€” so if you want to remain spoiler-free, don't read this!

Update: Babylon 5 creator J. Michael Straczynski responds on Twitter:

Update #2: Thanks to everybody who shared their knowledge and insights in comments. I'm really glad to see that this experiment succeeded in getting people to geek out about something they know more about than I do. I've always liked Babylon 5, but now I feel like I know a lot more about what makes it so important to so many people. Thanks a lot!