Krishna said (Text 16) that Action and Reaction is a subject that bewilders even the learned. Here he elaborates, stating that the subject remains a difficult mystery even to those who understand the three aspects of Karma (action and reaction).
Karma - Proper action
One fulfills ones desires in accord with scriptural regulation.
Vikarma - Improper action
One fulfills ones desires in a means that violates scriptural regulation.
Akarma - Inaction
One does not act to fulfil his desires.
Of the three, akarma (inaction) is the goal. In the upcoming verses, Krishna will explain how to achieve inaction.
Note: Although there are three aspects of action mentioned here, the remaining verses deal only with two: action and inaction. Both proper and improper action may be categorized as "action" (karma); and contrasted with inaction (akarma). The diagram explains.