From the Introduction:
The new Urban Arcana Campaign Setting expands on many concepts found in the d20 MODERN Roleplaying Game. First and foremost, it takes the 15-page campaign model presented in the core rules and transforms it into a full-fledged roleplaying world. URBAN ARCANA provides details on the history, denizens, and fantastic aspects of a world that would be very similar to our own if monsters roamed the sewers, magic was real, and your neighbors included elves, dwarves, and other familiar fantasy races.
Just as importantly, URBAN ARCANA adds several new aspects to the d20 MODERN rules. Here you can find new advanced classes, equipment, spells, creatures, and NPCs that can enhance any d20 MODERN game, whether or not it is based in the URBAN ARCANA setting.
URBAN ARCANA also introduces a concept that is already familiar to players of DUNGEONS & DRAGONS and other d20 system games - prestige classes. In fact, we found ourselves so inspired by the possibilities for different character archetypes that we soon had more prestige classes than we could possibly fit into the allotted space in the book. Rather than let the extras languish, we decided to offer them as a web enhancement.
Like those presented in the URBAN ARCANA book, the following prestige classes have requirements that are more restrictive than those for advanced classes, reflecting far deeper specialization. In general, prestige classes are accessible only after a hero has gained 8 to 10 character levels, and they often have requirements found only in advanced classes. These prestige classes were designed to interact with the other material in the URBAN ARCANA Campaign Setting, but they can be used in any d20 MODERN campaign.
Mastermind: This brilliant planner thrives on knowledge. She has plenty of minions to do her bidding while she sits at the center of her web and schemes.
Shapeshifter: A hardy hero with the ability to change form, the Shapeshifter can imitate others.
Sniper: The Sniper is a highly professional firearms expert who is skilled at making single, devastating shots.
SpecOp: An experienced, seasoned soldier, the SpecOp has greatly improved combat skills and capabilities.
Spellslinger: This magical gunslinger can make firearms disobey the laws of physics.