This game could definitely use some defense/resistance/damage clarification. I have no proof of what I've learned other than the fact that I played around with some different gear and tested a few things. I encourage others to post any other information they come across, whether it is proven fact or just their own observation. Corrections to my post are fully welcome, as I am not 100% positive of it all. Hopefully this can provide newer players with the knowledge to understand the gear statistics in Spiral Knights.
For anyone who isn't sure how to check your characters overall stats, hover your mouse over the bottom row of items in your character's equipment screen(default p key). This will show you the cumulative Defense/Attack Power and Resists of your gear, as well as any bonuses you have(examples: Damage bonus:low, Good chance of causing strong freeze.)
Also, keep in mind that the rating system of bars, is not truly accurate in most cases. From what I have heard, each increasing 'bar' is worth more than the last, and additionally, your characters overall power is relative to the tier/depth you are in(for example, my 3* sword does around 90 damage on level 15, but only 30 damage on level 3.) Please note that I'm simply using the number of bars as a measurement for my explanations because, well, there's no other way of representing an items stat value.
-Shield's status resistances are only effective when blocking. From my testing I've decided that you CAN in fact get status conditions such as poison and fire when you are blocking an attack(although it seems to be a much lower chance), unless your shield has the respective resistance stat(all shield resistances are absolute from my understanding). This is the concept I am most unsure about... I'm certain that the shield status resistances do not grant you the respective protection when you are NOT blocking(having the icebreaker shield with full freeze resistance will not make you immune to being frozen while you are not blocking), but I've yet to fully determine how they DO work.
-Shield's defense power does not seem to affect your overall defense power like your armor and helm. I can't say for sure but I'm thinking that it provides additional protection for when you are blocking(so your block isn't broken as quickly).
-Shield's health is how much damage you can take while blocking, before your block is broken.
-Armor and Helm defense power(ie. normal/elemental/piercing/shadow) are AVERAGED. If I have a helm with 10 bars of normal defense an armor with 12 bars of normal defense, my overall defense is 11 bars. If that helm also provides 10 bars of piercing defense, and the armor has 0 bars of piercing defense, my overall piercing defense is 5 bars.
-Armor and Helm resistances are ADDITIVE. If your helm provides 2 bars of shock resistance, and your armor provides 3 bars of shock resistance, then you will have an overall shock resistance of 5 bars.
-Weapon's attack power can be very confusing, since it seems the bars are not a very good representation of their true damage output. All damage numbers I am using are not accurate, they are just for representation purposes... If a sword does 10 bars of piercing and 10 bars of elemental, that basically means the damage is 20 bars, split between piercing and elemental damage. Factor in the enemy strengths/weaknesses and you will get your damage number. Lets say the enemy is strong against elemental. Your normal damage would remain 10, but your elemental damage would be halved to 5. Your damage would then be 15. If your enemy is strong against piercing AND elemental, then your damage would be 10. Enemy strengths HALVE the damage. I have not been able to determine how enemy weaknesses affect your damage, but I'm almost certain it does not double your damage, but rather does +50% more damage for that type.(I would love some clarification on this matter if anyone could provide it.)
-All bonuses provided by any gear(ie Damage bonus:fiend, Moderate chance of causing strong fire) stack and work towards your characters overall stats. Having a wolver coat(Damage bonus:low) and a wolver cap(damage bonus:low) will provide an overall Damage bonus:Medium. To take it one step further, if you're using a slime slasher(Damage bonus vs. slime:high) to fight slimes, you will get the Damage bonus:medium from your wolver set, PLUS your Damage bonus vs. slime:high.
I highly recommend browsing the SK wiki if you have any questions about this post or any core game mechanics.
I hope this wasn't too long winded or confusing. I tried to explain the concepts as clearly as possible for the noobs because when I first started, I was utterly clueless about how the stats worked for defense/resistance/damage, and I was unable to find any info about it. I could definitely be wrong about a few things so I encourage anyone with more knowledge to post in a respectful manner. Thanks, I hope this was helpful!
Minor thing to note:
Armor and helmet defenses are actually additive. The defense meter on the character page is just worth twice as much defense per bar. You can see how this works when you equip an Ancient Plate Mail with your 0-star helmet-- the armor's Normal defense shows one full meter, but the character's Normal defense goes way past half the meter (Ancient Plate Mail gives enough Normal defense that it actually overflows about half a meter past the normal armor meter).
For anything that's NOT Ancient Plate set, though, it acts almost exactly the same as averaging the two defense values-- since you're adding them together and then dividing the result in half to fit it into the character stats meter. It just works for stuff that overflows.