My understanding:
Higher con means you're more likely to gain more max HP when you level up. It's also used to determine your carrying capacity.
Strength adds to your damage - I believe weapons roll damage from 0 to max, and higher str means you roll closer to max more often.
Agility does the same thing for ranged weapons, and adds to defense value slightly.
Intelligence is the power of mages, as well as several skills.
Divinity is the power of priests, and a few skills.
In general, you want to raise con relatively quickly, to something like 20 or 40, depending, then raise str (for warriors) to a similar level, then a bit in agility (for warriors). If you have a lot of con vs str then you've got more stamina, you can survive more fights for longer. With higher str than con, you can hit harder, but will generally have to fall back sooner, or bring some powerful priest support.
Archers are one of the most powerful classes, as they automatically attack twice per round, and can benefit from Berzerker Rage, which greatly increases damage and isn't related to any attribute. A party of 3 archers, 1 warrior, and 1 priest isn't a bad idea. I'd likely raise them 2-and-1 for str/agi/div and con. That is, every level add 2 to their main attribute, and 1 to con. Anyone dealing damage needs Berserk Rage for sure, it's a godsend. Your warrior needs a shield, which means Shield Bash (I raise both rage and bash at the same rate). Your priest should probably go double shield - yes it does stack defense value - and mainly raise Awareness (spots traps and whatnot in dungeons) and Recall. I can always use all of my heal casts every day, so more casts isn't awful. You may get better luck out of other skills, but Recall is simple enough.
Later you'll want to hire a mage, and raise their Tactics and then Leadership, while just raising their Int. He'll be your mayor later. A lot of enemy groups will flee, making it hard to gain gold and finish quests, and Tactics makes it more difficult for them to flee, and Leadership increases your tax rate.
You may also have luck running 5 priests, going 1-1-1 on str-div-con. Whenever any of them starts getting hurt, any of the others can cast heal. Just sword and board them and go heal tanking through dungeons.