What exactly is chaos damage in Path of Exile? Does it bypass all resistances? Is it poison damage?
Chaos damage is poison in some cases, but not always. Chaos damage ignores armour, elemental resistances and energy shield. The only way so far to have resistance (in this case immunity) to chaos damage is by getting Chaos Inoculation from passive skills tree. If you have high evasion you can dodge some chaos attacks (like the "Viper Strike").
With the release of Open Beta, several forms of chaos resistance now exist in addition to Chaos Inoculation, including several nodes, armor affixes, and chaos resistance flasks.
It's another damage type, apart from elemental. Reduced by resistance to it, similar to elemental, going upto 75%
What's special about it:
a. It bypasses energy shield, and hits life direct! (hence chaos innoculation).
b. Chaos resistance is generally hard to come by. Much rarer on tree and gear. Currently no chaos resist auras like the elemental purities. This is true on monsters as well. Chaos resist is rare on monsters and no chaos resist map mods.
c. Arguably the most important - it cannot be reflected! There's elemental and phys reflect, but no chaos reflect.
It's not "poison" but Poison effects deal chaos damage. Most meta builds today utilize poison scaling btw. Triple entente - chaos/gem scaling double dip, no reflect and very little monster resistance.