It’s a debuff you put on enemies that causes them damage when they use skills. Not so great in PvE, but awesome in PvP when people don’t notice.
The chaos armour you get from ethereal combo fields is the result of a combo finisher and doesn’t need to be spelt out. The tooltips for combo fields would be unwieldly if they had to describe what effect each finisher has (e.g. leap gives you chaos armour, physical projectile inflicts confusion etc.). Experiment and see what things do, or just google it:
http://wiki.guildwars2.com/wiki/Combo