There are many different resistances. The main four are Ranged, Melee, Magic and Physical. Note that Melee and Ranged resistances are also considered Physical damage, and thus are effective against Rouges and Warriors. Magic Resistance is primarily used to negate damage from Mages. Not that the bonuses from Cm's aren't show on the stats on the loadng screen or in the battle. They are included in damage calculations but aren't easily known.
See here for a special resist designed for ai.
Elemental Resistance: There is one variety for each of the elemental types of damage. While the main resistances are effective in a general sense, elemental resistances are applied for more niche uses. For example, Arcane resistance is very useful in brawls Mages use their basic Arcane Shot. The different resistances include Arcane, Fire, Lightning, Ice and Nether.
Stun Resistance: Helps you not get stun locked, although speed can also add to recovery from stun. Other resists can lessen the effects of stun. This is particularly useful against glass cannon builds who primarily use stun.
Other types of effect resistances include poison, burn, and bleed resist. Although not widely used, they can be useful against effect builds who depend on stacking these effects to win. A warrior can use the skill: Bandage Wounds to get 50% resistances in these three effects, so it is not recommended to invest in these resistances for warriors.
It's important to note how stacking resist will effect the overall damage. Without getting too complicated on the damages with elemental bonuses on physical attacks being reduced with magic resists. Will keep the main source of the damage. Also ignored armor.
So starting with the physical classes. With melee/ranged resistance being used for ranged and melee types.
First note that 50% melee/ranged res and 50% physical res aren't added together but are multiplied. So the resuilt of the total damage negated is 75%.
10% on melee/ranged res with 10% physical res = 19%
20%, 20%= 36%
30%, 30%= 51%
40%. 40%= 64%
50%, 50%= 75%
60%, 60%= 84%
70%, 70%= 91%
80%, 80%= 96%
90%, 90%= 99%
100%, 100%=100% (also note here that if only one resist say physical is 100% it will block out practically all ranged and melee damage. Burn attacks can hit and the elemental bonuses but basically armor will negate whatever it left.)