Wizard Spells and Damage guide
Diablo III Weapon Science Guide
Weapon Damage Calculation Video
Diablo III Damage Calculator
Wizard and Witch Doctor weapon damage system explanation submitted by trog:
Each weapon has three relevant base stats:
The tooltip also displays average DPS, which is actually a red herring: the min/max/AS numbers are much more relevant. If you have a 1.0 AS staff that does 20-30 damage with 20 intelligence (and you have no other int), that means that each hit will roll a random number between 20 and 30 and then that number will be multiplied by 1.2 (remember, each point of int is +1% damage).
Effective Damage Range: The effective damage range of that weapon for a Witch Doctor or Wizard is actually 24-36. Your "Damage" stat in the character sheet factors this in and spits out an average DPS stat accounting for all modifiers: in this case, you'd see a damage stat of 30. Of course, the damage stat is just a guide: whenever you use a skill, it grabs a random number from the damage range and then applies all bonuses and modifiers-- the Damage stat has very little correlation to the size of a single hit!
If you had a different 1.0 AS weapon that had a damage range of 25-37 but no Int, it would do more damage because the base DPS is more than 20% higher.
This is also why those +1-2 damage rings are so good in early Normal mode. A normal one-hander blue before level 20 is gonna rock something like 18 DPS (example:17.5DPS 10-17 damage range 1.3AS Axe). If you're using that axe and you slap on a pair of 1-2 damage rings, it goes to 12-21 damage or 21.45 DPS, albeit with higher variance than before. Those crappy little rings are worth 22 intelligence in this scheme!
This system is also why you such strange outliers in terms of DPS. Ceremonial Knives always seem like they've got insane DPS, right? Well, they do, but they also have stupid high variance. A white level 24 Ceremonial Knife has a damage range of 10-37; a comparable level 22 mace has a damage range of 18-32, meaning that hits will be much more consistent.
Witch Doctors with Knives probably notice that sometimes a Firebat will one-shot foes and sometimes it'll half-health them-- it's thanks to the variance. In general, high variance is fine on fast, cheap attacks, and awful on slow, expensive ones.
Here is another guide about weapon damage. Thanks to qp0n for the following guide:
Figuring out the basics is easy once you figure out the damage multipliers.
There are only two things that get tricky for the average player:
1 ) Flat +damage stats will always add identical total damage to your 'weapon damage', but the DPS added depends on the weapon speed. Adding +10 damage to a 1.50 dagger will add 15 DPS because that 10 damage is applied 1.5 times per second, whereas a 1.00 staff would only gain 10 DPS, despite both weapons gaining exactly +10 weapon damage.
Spell damage won't be changed however since it is calculated using weapon damage not DPS. The difference in DPS is made up for with casting speed.
2) Faster weapons are not necessarily better than slower ones, and vice versa. The 'weapon damage' that spells use to calculate "X% of weapon damage" is not using your DPS but your actual damage per 'hit'.
If you have two weapons like the two mentioned above and both have exactly 90 DPS, but at 1.50 and 1.00 speeds respectively... the former will have 60 weapon damage and the latter 90 weapon damage. Meaning spells using the staff will do more damage, but the dagger will allow the casting of successive spells 50% faster.
For #2 the tradeoff comes down to your Arcane Power pool and cooldowns. There technically wouldn't be any difference between the two weapons in a straight resource-free, cooldown-free nuke test ... but since your AP is finite and some damage spells have cooldowns, you may want to get the most damage out of those casts rather than the ability to cast quicker.
In that case the slower weapon will deal the most damage. Or you may prefer faster casts to proc any number of things such as AP regen on hit. It's all preference.
(Personally I prefer the slower the better, biggest bang for the buck. So don't laugh if I join your group carrying a 2H samurai sword!)
Wizard Attack speed Test