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.
Sources: Official forum , PoE wiki
No, multiple instances of the burning ground damage from Fire Trap will NOT stack.
Only different sources of burning damage can stack. For example, an Ignite debuff from a spell's critical hit and the Fire Trap ground burn will stack.
When 2 of the same burning sources are present the highest damage source will take effect. Multiple effects can still apply, like 2 Ignites on one mob, but only the highest damage one will actually deal damage.
Once the highest damage effect ends, if another instance of that effect was applied and still has duration left, it will then do what damage it has left. This goes for Righteous Fire, Ignite, and Fire Trap's ground burn effect. Searing Bond, a recently added skill, is an exception and the beams can stack with itself.
This is why you should try to spread out Fire traps a little bit whenever possible.
Posted by Mark_GGG, PoE developer and creator of the games skills and their mechanics.
...Ignite, RF, ground fire and now Searing Bond are all separate
sources of burning damage, and all stack. Ignite, RF and ground fire
do not stack with themselves (only the highest of each takes effect at
a time), but do with each other.
Searing bond beams stack.
As of Patch 1.0.5 Searing Bond Beams no longer stack.
Best Answer
Status ailments in Path of Exile are based on the damage of the hit. For instance, in your case:
You can read up on the way damage calculations are made here:
Essentially, the "Chills/Shocks enemies as though dealing X% more damage" will calculate your skill's chance to chill based on an increased damage modifier of your damage dealt. Take a hypothetical scenario where you deal 100 cold damage to the enemy. A level 1 Ice Shot has a "Chilled enemies as though dealing 100% more damage". Instead of your chill effect being based off 100 damage, it would instead be based off the base damage "as if it was 100% more damage", thus making your chill effect based off "200 damage", despite you not dealing that much.
Basically, it increases your threshold to apply the chill/shock in the first place, since a higher damage value is being used in the calculation of that effect being applied.