Thyressa is a world caught in the throes of scientific and magical revolution, and alchemists are very good at advancing both fields. Alchemists are very often on the cutting edge of innovation, and they fulfill a very important role in Thyressa.
Ardwrights are like wizards, but they use their magic for more technological purposes.
Although many berserkers come from primitive tribes, not all do. Their rage ability and durability makes them useful shock troops, and many militaries specifically train people for such roles.
Bards in Thyressa fulfill much the same role as bards do in almost all Pathfinder games. Their place in the world is one that transcends societies and technology levels.
The cavalier class, and it's vassal archetype, serves the same role, with the exception that cavalry are usually well trained military or law enforcement officials recruited from civilian (almost always commoner) volunteers, not nobles trained for birth from war.
The gods of Thyressa are very mysterious. Their existance is well known, though whether god is the correct term is not, and their personalities and goals are an enigma. The fact that corrupt clerics do not lose their powers further complicates this. Aside from the uncertainty of religion in Thyressa clerics fulfill much the same roles as in any other Pathfinder game.
Throughout most of history, divine magic has been seen as the only pure and acceptable magic, and arcane magic, mostly in the form of witchcraft, as something not to be trusted. Clerics served on the front lines of this war against arcane magic and it's users, and only within the last several decades has the war cooled down. Many Clerics, however, are still vehemently opposed to arcane magic.
Druids in Thyressa are stalwart guardians of nature, but they are not necessarily opposed to civilization or technology. While some certainly do loath both of these things, others are accepting of them, believing that nature and civilization can coexist if people are willing to make an effort to be responsible. It all depends on the individual.
Fighters in Thyressa are very similar to fighters in other Pathfinder campaign settings, as everybody needs warriors.
Guns are a very new thing in Thyressa. They are expensive, complicated, dangerous, and seen as being inferior to bows. That said, the weapons have their lovers, and a properly trained gunslinger can be deadly in a fight.
Inquisitors in Thyressa fulfill much the same role as inquisitors in other Pathfinder games, and the uncertainy of religion and the precise will of their diety makes Thyressan inquisitors rather dangerous. Though they can be good aligned hunters of threats of divine origin and corrupt clerics, they are more often brutal and tyrannical enforcers of a corrupt and greedy religious order. Like clerics, corrupt inquisitors do not lose their powers.
Historically, inquisitors have served alongside clerics on the frontlines of the war on witchcraft, and although arcane magic is becoming more acceptable, some inquisitors, just like some clerics, still cling to the old ways. Witch hunting inquisitors can be a major threat to arcane spellcasters.
The Magus fills a combination of the role of the wizard and that of the warrior.
Monks in Thyressa are similar to those in other Pathfinder campaigns.
Most oracles in Devaia struggle to understand what dieties chose them and why, but otherwise function much like clerics, often but not always with a divination focus.
Paladins in Thyressa fill much the same roles as they do in any campaign setting, except that not all are intrinsically lawful or worship a specific deity. Paladins who fall slowly gain Antipaladin powers, and a fallen Paladin who still believes herself to be on the path of good can be a very dangerous thing.
Pioneers serve a great many roles in Thyressa. Some are guardians of nature, some are scouts, some are explorers, some are archaeologists, some are hunters, and some undertake other roles I haven't thought of. They are an incredibly diverse bunch, with only their wilderness and combat skills linking them together.
Psionics are a distict branch of arcane magic that has it's own mechanics. It's practice is about as old as wizardry and treated about the same, but psionic users are the least common arcane magic users.
Rogues in Thyressa fill the same roles as in any other Pathfinder game. The agent alternate class is treated the same as the rogue class.
Sorcerers have always existed in Thyressa, though the utility of their powers was somewhat limited until around 80 years ago, and do to the frequent witch hunts which failed to distinguish between witch and sorcerer they mostly stayed hidden. This has all changed with the growing usefulness of arcane magic and the end of large scale witch hunts, and sorcerers are finding themselves both more powerful and more socially accepted. Since sorcerers have historically been much less powerful than they are in the modern day, there is little in the way of past knowledge or community for them to turn to in order to help understand their powers. Most are on their own in this regard.
Summoners are simply another type of wizard in Thyressa.
For hundreds of years witches have been the most common arcane spellcasters in Thyressa. Due to the long casting times of spells up until around 80 years ago, the effects witches produced, which do not require rapid casting to be effective, were the most useful of all arcane magics. However, being the only arcane spellcasters around aside from the occassional sorcerer had it's drawbacks. Arcane magic was seen as something not to be trusted and divine magic as more pure, a belief only now fading away, and therefore witch hunts were highly common. Do to this mistrust, witches have always been a rather secretive lot, careful not to tell outsiders of their ways.
Wizardry is a very young art in Thyressa. Up until around 50 years ago, arcane magic simply took too long to cast for any type other than witchcraft to be practical. That all changed with the discovery of different casting methods that made casting quickly feasible, allowing the study of wizardry to suddenly have a practical use. The study of wizardry is in it's infancy, and wizards are still figuring out what is and isn't possible. There is very little old knowledge to fall back on, as wizardry is only about five decades old, so innovation is key.