Attacker takes damage of x?

[D-Dave]

As far as I know:

"Damage reduced by x" (and x being a plain number) it does just that, it reduces incoming (physical) attack's damage by the number listed.

(in contrast to damage reduced by x% which acts like a physical resistance)
Worth traking note in this context is the order of which damage is affected by those affixes:
[well, according to elric the order below is not true for phys attacks (see his post above) and I am sort of willing to trust him on that one ;) ]
first: plain reduction (damage reduced by x (phys) and magic damage reduced by x (elemental and magic))
second: resistance (damage reduced by x% (phys) and resist fire/coldghtning/poison x%)

"Attacker takes damage of x" on the other hand triggers a form of an attack from your character on the monster hitting you (does not work against ranged attacks) which is in turn affected by monsters resistance.
It can (simplified) be seen as an effect like the paladins "thorns" aura not taking into account how much damage an attacker does.

Hope this helps :)

edit: ok, ok I am just too slow typing....elric beat me by 3 minutes.....