The Centauri homeworld has been seen in the second season episode: "The Coming of Shadows". It appears to be a relatively Earth-like planet. It seems to possess oceans and landmasses in similiar ratios to Earth. The sky is blue, indicating a relatively dense atmosphere.
Centauri males use the length of their hair fans to indicate social status. They also have six penis-analogs. Centauri females shave their heads.
The Centauri Republic was the first formal contact the Earth Alliance had with another alien government.
Centauri ambassador to Babylon5, Londo Mollari, has become involved with the Shadows through a third party, Morden.
Centauri technology seems to be superior to Earth Alliance technology and inferior to Minbari technology.