An activity is initiated by some motive, follows some purpose, and may occasionally bring about some consequences. An action assume some reason and is aimed to something; one can complete an action and obtain a result. An operation satisfies some need and is intended to have a definite effect. From the viewpoint of the hierarchical approach, an activity is a hierarchy of actions falling within its scope. Inversely, each action is a stage of some activity, having sense only as an element of its hierarchy. Under different circumstances, this hierarchy may unfold in different ways, providing one or another possible realization. In fact, any action may refer to many activities, one of them being brought to the top of the hierarchy in the process of motivation. In the same way, the meaning of an action is the hierarchy of possible operations, unfolding itself each time into a different implementation. An operation is a means for some action, and the place of an operation in the hierarchy of action defines its function. Likewise, the process of rationalization selects one of the many actions implying the same operation.