About Stats and What They Do
In this post I'm going to go over the different stats in game along with the extra boosts each one gives. Some of this isn't known by new players, so hopefully by knowing it early you can better plan what to do with your training points.
Strength
This stat is pretty self explanatory. It makes you stronger! This allows for a few things. If you don't have a weightless (if you are serious about your altering you should get one) then it allows you to carry more weight. This is important because if you have too much weight, then your encumbrance is high, and you will lose movement a lot faster when walking around. This means that it can be useful for non melee classes, but this can be avoided by buying a weightless. If buying a weightless isn't an option, then be ready to put points into strength.
If you are a melee fighter, then your weapons are going to have a minimum strength requirement to wield. Shields will also have a minimum strength requirement. For this reason both warriors and thieves have to train strength to use their weapons and held items.
In addition to the ability to wield weapons and shields, strength also adds damage in the form of DR. This is 1dr for every two strength past 28. This really doesn't add up to a bunch though. So if you have your strength completely maxed out at 34 you'll get a +3 boost to your DR. It isn't a game changer, but could be nice nevertheless.
There is a mage strength spell that gives you a boost to your STR. This can be added to your recast, and stacked to give up to +5 strength.
Intelligence
Intelligence is a requirement for every character, to one extent or another. Even if you are a warrior then you are going to want at least 24 intelligence so you can cast displacement at level 19 mage. Intelligence is used as a primary or secondary stat for mage, necro, druid, thief, and for a few high level cleric spells. With mage and necro getting equipment with intelligence is pretty easy, but as druid it can be quite difficult, and the requirements for the druid spells is quite high. For example the level 30 druid spell solar flare requires 29 intelligence. That is no problem for a necro or a mage, but it is a headache for a lot of druid characters. This is something to keep in mind as you are leveling up.
If you are a thief this is probably an annoying stat. Not only do you need to train strength for weapons, and dexterity for not dying in combat, you also need intelligence for many thief skills. This is one of the many reasons thief is a difficult class.
Outside of spell and skill requirements. Intelligence is quite important for some crafting. For example, if you try smelting a more difficult metal with low intelligence, then it is going to fail and your ore will be destroyed. This is why it is good to have a luck set for crafting made with electrum and green jade. Jade gives the luck, and electrum gives the intelligence boost.
Intelligence can also give some mana towards your mana reserves, but it is so little that it isn't going to make a huge difference. It may be attached to your mage level the same way constitution is, but at the time of writing this I have not tested to see if this is the case.
Wisdom
Wisdom is an important primary or secondary stat for cleric, druid, mage, and necro. This is true whether it is your primary or secondary class, and this is one of the reasons the warrior cleric class option can be so frustrating. Having to train the four warrior stats of strength, constitution, dexterity, and charisma, you then need wisdom as well. For those classes that require wisdom for casting spells, it isn't a hard stat to get. Necro and druid have an especially high amount of wisdom on their equipment. Cleric and mage seems to have enough which is good, since every thing else about mage and cleric equipment is horse crap.
Wisdom also helps with total mana, and mana regen. It does seem to have a noticeable effect on regen, to the point where training it enough can actually boost your default regen rate. This is nice for a caster for sure. According to the help file in game (help 1.wisdom) it also helps with not being stunned or dazed. I don't know how true this is. Even with 26 wisdom on my warrior character I'm getting dazed. If possible it's probably better to rely on valor to avoid the dazing.
Dexterity
Though thief is the only class that has a dexterity requirement, every class benefits from increased dexterity. It is the most important for thief and warriors though. With thief skills it is a requirement to have dex, but with warrior stuff like shield block is more effective with a high dex.
Dexterity is the forgotten stat. At level 2 thief you can learn the skill dodge, and this in combination with a highish dexterity means you will get hit a lot less in combat. If you can get your dex up to 25 then you will notice a lot less damage taken in combat. This is an important stat for every character in game, especially in higher level areas.
Increasing your dex also means you can carry more items in your inventory, and this is always good. Being able to carry over 15 items is pretty great, and it can make your life a lot easier. You can also learn the mage spell dexterity to give you a boost to your dex. It can be added to recast and it stacks up to +5.
Constitution
At the point of writing this constitution isn't a required stat for any classes for learning skills or spells, but to keep from dying it is quite necessary. It is easy to forget about it since it isn't required by anything, but this is unwise. Con effects quite a bit of things for your character, some of which are HP, HP regen, movement, and move regen. These are extremely important for a warrior and thief, but also quite important for all other classes. As you start going to higher level maps, having more HP is very necessary. With the base of around 500 you can expect to die quite fast.
In addition to the hp boost from con, you also get a boost from leveling warrior, and the boost is related to the con amount. The more con you have the more hp you get from leveling warrior, and the higher your warrior level the more hp you get from training con. No matter what it works out to the same amount, so training con up to max then leveling warrior won't result in more hp. This is why it is good to always have warrior as your fourth class. This way you can easily get to 1k hp, while still having your top three classes be something you want.
In addition to the hp and movement boosts, constitution also has an effect on certain warrior skills like second wind. With a higher con your wind recharges faster allowing you to use it more often. The help file says it also helps prevent being dazed and resisting poisons, but I cannot say how much this is true. I have maxed con, and I still get dazed and poisoned. It is probably better to on valor to avoid being dazed, and the cleric spell slow poison to reduce poison attacks.
Charisma
This is a primary stat for the clerics, but a non listed third stat for other classes. You'll need chr if you want to do cleric things, but you'll also need it for decent cry of victory as a warrior, to be successful at stealing souls as a necro, and for talking your way out of being caught stealing as a thief. It is also listed as being useful for druid with controlling animals, but I don't know anyone who actually does that. So though it is listed as a requirement for cleric spells, it is also a requirement to properly play other classes.
With necro the higher your chr, the more likely you are going to have a successful soul steal. With warrior you will get more hp and movement back after a kill by using your cry of victory skill. With thief you can use fast talking to talk your way out of getting caught stealing, and this can be very useful in high level areas where you'll get killed dead.
One of the most important things about chr that isn't talked about is how it effects who gets hit in a group. This is very important for the necro and druid class. You want to have a higher chr than the minions in your group, if you do not then you will get attacked more than they do, and this will be ungood for you. With necro zombies are a good punching bag, but if you have lower chr than your zombie you are going to end up being the punching bag. A high chr in combination with dodge and a high dex should keep you from taking too much damage on most maps.
In addition to this, having a high charisma means that aggressive animals will either down right fail to attack you, or they will hesitate. This is great if you want to get in the first hit, or if you are warrior or thief and want to charge or backstab. Also with the help of sneak it allows you to walk through some aggressive areas without being attacked. You can also get a small discount at NPC shops if your have a high chr. It isn't a crazy amount, but who doesn't like to save some gold in shops.