The series is set some 250 years in the future, when Earth has significant spaceflight capabilities and is moving out into the galaxy to colonize. Humans encounter several other powerful sentient races in the galaxy, as well as many less-powerful races. The realities of politics exist even in the future, as much of the series deals with Earth establishing itself as a major power within the galaxy and must interact with the other races. The Babylon 5 station was built in neutral space in order to encourage peaceful relations and trade.
This series was quite remarkable for TV science fiction of its time (the 1990s) because it was very serialized and each episode was not self-contained. The build up of the overall plot of the series is a pleasure to watch, and it's nice to see events that should have long-term effects actually having those effects.
Keep in mind that all my reviews have spoilers. This is more significant that in most of my series reviews, because I will comment on how episode events will affect future episodes. If you haven't seen the series before, you may not read these reviews yet. I strongly encourage you to watch the whole series "unspoiled" because it's worth the trip.