Hit Chance is represented as a percentage, and it's the chance that you will actually land your skills on your target. It comes in two flavours, Physical and Magical. Physical Hit is used for the majority of melee class skills, with the exception of the Myrmidon that also has magic skills, and Magical Hit mostly for caster class spells.
By default, you have 100% Physical and Magical Hit Chance, even without any items equipped - you can see this in the Hero Info window when you unequip all of your items, you still have 100%. This means you have 100% chance to hit your target (player, pet or monster) of the SAME level that has 0% Evasion.
Evasion is when you completely avoid being hit by an attack or spell - think of it like dodging, or in the case of spells, resisting them.
If the target has 20% Physical Evasion and you have 100% Physical Hit, then you will then have an 80% chance to hit them.
So, to ALWAYS hit a target you need to have 100% more Hit Chance in your Hero Info window than they have Evasion in theirs.
Q. Why do I sometimes see "Resist" when I use an attack, even though the skill appears to have done it's damage?
A. The skill has has a debuff, and even though the skill itself hit the target, the debuff was resisted.
Q. Why would a debuff be resisted when the skill itself hit the target?
A1. The target is immune to the debuff.
A2. Debuffs have their own % chance to land on the target and it's usually around 80% but depends on the skill and rank of the skill. You can see the chance of the debuff landing in the skill's tooltip. Also, your target may have Resistance stat that reduces the chance of debuffs landing on them. If they have 10% Resistance it will reduce your debuff's chance from 80% to 72%.
Q. Can I somehow increase the chance for debuffs to land, with my Hit Rating or something?
A. Yes. There is a secondary stat available on some items called Accuracy Rating. Accuracy increases the chance of your debuffs.
Q. Does my level and the target's level affect the chance for the debuff to land?
A. Yes. If you are a lower level than the target, the chance is reduced by upto half and if you are a higher level than the target it is increased by upto x1.4.
Q. Can I somehow increase the chance of avoiding a debuff, even if the skill itself hit me?
A. Yes. You can use skills, potions, transformations, wearing cards and items with chance effects that immunise you against certain types of debuffs for a period of time and you can make use of the Resistance Rating stat that will reduce the chance of debuffs landing on you.
Q. What is this "Efficiency" I see on skills, potions, transformation cards etc.?
A. Efficiency is like the rank of the buff/debuff and is used when deciding if another buff/debuff can overwrite it, or if a certain type of immunisation or skill can remove it. For example, if a debuff has an efficiency of 6, you would need to use an immunisation or skill that removes debuffs of efficiency 6 or higher to remove it.
Q. Why are there debuffs in the game with really high efficiency and nothing that can remove or prevent them?
A. These debuffs are set with such high efficiency because they are not meant to be avoided or removed in any way, to add difficulty to the game and to ensure certain things happen, such as having a healer, or making you use certain items. You can usually tell that a debuff cannot be removed because it's icon is grey. Red debuff icons are Physical, Blue are Magic, and Green are Poison, Grey can be any of the three and cannot be removed.