To add to what Crothian said:
The three options he mentioned are templates that can be applied to any class, each also has a 1-2 level class that let's you expand your abilities within the template.
There are six 10 lvl basic classes...Aristocrat, Entertainer, Investigator, Scholar, Scoundrel & Warrior. There are also nine 5 lvl prestige classes...Ace, Barbarian Lord, Disciple, Explorer, Inventor, Mercenary, Occultist, Spy & Mastermind.
It is sort of an odd duck as it is closer to D20 Modern in feel but the 3.5 PHB is listed as the "required" source (no doubt because there are a lot more PHBs out there).
The background feats are absolutely great and the new "daring" feats do an excellent job of imparting a pulp feel. The Inspiration Points/Dramatic Editing rules are IMO the coolest iteration of Action/Hero points out there.
While Adventure! D20 works well enough as a standalone product I think it really comes into it's own if you yank the Templates/Feats/Powers/Inspiration Points & use them on top of D20 Modern or Grim Tales.