From the publisher's website:
This definitive sourcebook for the Pathfinder Roleplaying Game provides tons of new character options for all seven “core” player character races, from archetypes that allow elf characters to explore their connection to nature and magic to feats and spells that let a dwarf character carry on the legacy of his multi-generational clan or a gnome explore her connection to the First World or delve deep into her weird obsessions.
Additionally, the Advanced Race Guide offers meaty sections on a dozen “spotlight” races that make interesting and exciting player character options, such as goblins, aasimar, tieflings, dhampyrs, drow, the elemental races from Bestiary 2, and several others.
All PC-appropriate monster races in the Pathfinder Roleplaying Game, including creatures from all three Bestiaries and The Inner Sea World Guide, receive race options equivalent to those presented for the core races in the Core Rulebook, for the first time allowing players to create and play characters like merfolk, grippli, duergar, stryx, and every other appropriate monster currently in the Pathfinder game.
Lastly, the Advanced Race Guide includes an extensive section that allows players and GMs to build their own custom races, either to emulate more powerful creatures that already exist in the game or to create wholly original characters unique to their campaign. This section will be the focus of Paizo’s next major Open Playtest effort, which will kick off in late fall 2011.