Mages are mages. Their talent trees are: 1) DPS spec - more damage (seems to also currently include weapon buffs on others and themselves - all increases the party's DPS) 2) Crowd Control spec - status effects 3) Buff spec - buffs (mostly on themselves)
Priests are priests. Their talent trees are: 1) Healing spec - takes First Aid, Healing/Uncursing? 2) Mage spec - DPS and crowd control, Smite Body/Smite? Soul [Shadow] 3) Buff spec - buffs (mostly on others) Valour/Blessing?/Undead? Creation [Discipline] (There are also the 'interesting' miracle trees - Mind Control, Cursing and Knowledge - which aren't directly combat-applicable mostly)
Rogues are rogues, mostly. Their talent trees are: 1) DPS spec - more damage 2) Stealth spec - sniping, fingerscouting 3) Survivability spec - DAC, Light Armour
Wilderness are hunters, but they don't get pets. Their talent trees are: 1) DPS spec - more damage 2) Skirmisher spec - DAC, Light Armour, status effects by weapon 3) Mage spec - status effects by bow
(Rogues and Wilderness have many interesting abilities, like forgery and tracking, but they're not relevant to combat)
Alchemists are, um, maybe Shamans actually - they have lots of interesting useful buffs, they can successfully specialise in being the party healer if they like, and they can also not die immediately, all they're lacking from Shaman is the ranged damage. (Splash Wilderness for bow comp and you're sorted.) Their talent trees might be: 1) Healing 2) Venoms 3) Buffs
or they might be
1) Volume 2) Power 3) Research