All characters’ race will be the first five of the six Kindreds (i.e. not orc), plus the two hybrid forms. In other words the core races covered in the Pathfinder Core Rulebook. Some class/race combinations are not allowed (or will require a very special backstory, which must be discussed with the GM).
You can peruse the Glossary entries for the backstory flavor of the individual races. Some specific points to remember:
A gnome will never dedicate himself to a particular Power, and consequently there are no gnome clerics.
All dwarve characters are male. There are no dwarvish arcane casters (wizard, sorcerer, or bard).
A player character must select which of the six Folk they belong to.
Elves can be any class, but they are distinguished by the fact that they are “immune” to any raise-from-the-dead magic. In compensation, they show no negative effects from aging (though they still benefit from the positive changes), and have no maximum age.
All player characters are from the same generation, that is, born during the twenty years or so after the Darkness, Some of these elves share a common gift name Annamornorto.