From publisher blurb:
The priestess is an essential part of every society. Volvas, oracles, prophets, and seers shape the fate of heroes and cultures. They advise peasants and kings, and confront demons and undead. Charismatic and wise, they command more divine power than any other spiritual profession. The priestess is an academician, a judge, and a shepherd who watches over her flock. In an adventuring group, she occupies a role somewhere between wizard and cleric.
Compared to the cleric, the priestess has a longer spell list and is more effective against undead. She can also inspire her allies with heart-felt sermons and inoculate them against some mind effects. Like a wizard, she doesn't wear armor or use many weapons. She relies on magic and lore to overcome obstacles, and she has some extra spells to help do that.
With the priestess, players can support their allies better than a cleric and they can blast enemies almost as well as a wizard.