From the back of the book (4th edition):
This supplement is a must for roleplayers! Walk with a Buddon priest seeking revenge. Watch him play his Death Cards. Chat with Phelinssarious Phentari, galactic fugitive at large. Or you might converse with the Goola Goola, the would be electronic genius.
The Galactic Underground provides you the gamer with three new player character races, new skills, and over 100 special powers! The future is yours to conquer.