From Diablo Wiki
Resistance is one of the basic character stats in Diablo III. As in previous games in the series, characters must use equipment and skills to build up resistance to various types of damage, thus decreasing the health they lose when faced with those attacks.
All resistances (including physical) in Diablo III act as a form of damage mitigation, reducing the amount of damage taken by a percentage. Higher values to resistances = more damage absorbed, with diminishing returns; characters gain more going from 0-100 resistance than from 100-200 resistance, and still less (in percentage mitigated) from 200-300, and so on.
There are six types of resistance listed in the Character window: Physical, Cold, Fire, Lightning, Poison, and Arcane/Holy. Damage mitigation is also provided by the Armor value, and character level vs. monster level factors in as well. Damage may also be blocked or reduced by other items, item modifiers, and skills.
There are no immunities in Diablo 3, either for characters or monsters.
 Resistance Diminishing Returns
A general guideline (all values for level 60 character vs. lvl 60 monsters):
- 50 Resistance = 14.29% damage reduced (16.67% EHP increased)
- 100 Resistance = 25.00% damage reduced (33.33% EHP increased)
- 200 Resistance = 40.00% damage reduced (66.67% EHP increased)
- 300 Resistance = 50.00% damage reduced (100% EHP increased)
- 400 Resistance = 57.14% damage reduced (133.33% EHP increased)
- 500 Resistance = 62.50% damage reduced (166.67% EHP increased)
- 600 Resistance = 66.67% damage reduced (200% EHP increased)
- 700 Resistance = 70.00% damage reduced (233.33% EHP increased)
- 800 Resistance = 72.72% damage reduced (266.67% EHP increased)
- 900 Resistance = 75.00% damage reduced (300% EHP increased)
- 1000 Resistance = 76.92% damage reduced (333.33% EHP increased)
- 1100 Resistance = 78.57% damage reduced (366.67% EHP increased)
- 1200 Resistance = 80.00% damage reduced (400% EHP increased)
In other words, the first 100 resistance is good for 25% damage reduction, while going from 700 to 800 only adds less than 3% more damage reduction. Of course that 3% might be very important for a high level character taking damage from numerous sources, but players need to balance the improvement they'll gain from higher resistance vs. adding more hit points, or higher armor, or better blocking, etc.
 Resistances Importance
Higher resistances are essential for character survival on higher difficulty levels in Diablo 3. While resistances aren't very necessary during the leveling up process, they become mandatory in Inferno, and especially past Act One of Inferno. Ranged characters who stay out of trouble can get by with fairly low resistances; 150-250 or so, though higher hit points may be required and such characters must be very careful about elemental attacks, especially from Boss Modifiers. Melee characters generally need much higher resistances, and most Barbarians and Monks aim for 500 or higher. Values above 800 or even 1000 are not uncommon for well-geared Monks playing on the higher Monster Power settings.
All types of resistance are valuable in Diablo III. This is a change from Diablo 2, where Lightning was by far the most important, with Fire next and Cold/Poison largely irrelevant. Though all resistances were necessary, prior to the v1.07 Patch Fire and Arcane were the most deadly forms of damage, largely due to the Molten and Arcane Enchanted Boss Modifiers. That patch nerfed Molten and buffed the Plagued modifier, making Fire less dangerous and Poison more useful.
Due to the wide variety of elemental damage types and resistances most players try to obtain items with the All Res modifier to cover all their bases. This situation is exacerbated by the item modifier design, which (inexplicably) grants higher values to Res All than to any of the individual types of resistance. Most types of armor, for instance, can roll up to 80 Res All but only up to 60 to any individual resistance. If the resistances could roll up to say 100 or 120 then characters might benefit from hunting for particular resistances instead of just Res All.
The one class that can partially sidestep this item modifier bottleneck is the Monk, thanks to the One With Everything passive. That passive skill boosts all of the Monk's resistances to the value of the highest single resistance, and lets each Monk hunt for items with Arcane, or Cold, or Lightning (etc) resistance to add up on top of the the Res All total.
 Monster Resistances
Monsters do not have resistances in Diablo III. They do not need them, since a fairly late change during game development was to remove any difference between the various types of damage players deal. Thus the elemental damages on weapons and skills are essentially just different colors of damage, and players never need to worry about using more Fire on a level with monsters that are low on resistance to it, or not using Poison against monsters with high resistance.
All skills and weapons work the same in PvM regardless of their elemental damage type. This is not the case in PvP and individual resistances do matter there, but since almost all characters stack up Res All rather than any individual type of resistance, the impact on play is is negligible.
 Development: Diablo III Resistances
As for immunities, we're not doing double/triple immunities (in monsters). We don't want to completely hurt a character's ability to hurt something. We also diversify damage types more across the classes (than was done in Diablo II). But we still can't count on a class having a diversity of damage types we can use.
Question: Will characters need to stack resistances for particular fights?Jay Wilson: Sometimes. It's kind of an end game question, so haven't done it yet. We'll look more at it as the time comes closer.
 Diablo II Resistances
Diablo II had six types of damage, four of which could be resisted by players. Lightning, Fire, Cold, and Poison. Magic damage could not be directly resisted by players, nor could Physical, though they could be reduced or absorbed.
Resistances maxed out at 75%, though they could be raised as high as 90% via some unique items. There was a 40% resistance penalty on Nightmare and a 70% resistance penalty on hell, forcing players to add better resistance gear to maintain their damage mitigation on higher difficulty levels. There were numerous skills and curses that could raise or lower resistances across the board, for both players and monsters, and high level characters often had their resistances raised far above the 75% cap, so they would remain protected even after being debuffed.
Monster damage types were unevenly-distributed in Diablo 2, and characters benefited far more from resistance to Lightning than to any other element. This is not the case in Diablo 3, where the utility of the various types of resistance are fairly even.