Decimation is one of the IO sets. It provides indivual bonuses plus set bonuses for the more pieces of the set you slot in one power. The abbreviations refer to which aspects of a power that enhancment improves; Acc = Accuracy, Dmg = Damage, and so on. The number before the parenthesis represent the level of the enhancement, enhancements provide a bonus based on their level. The number in the parenthesis is the level at which the slot was taken, the (A) stands for the initial slot that all powers get when chosen.