Activities are composed of goal-directed actions that must be undertaken to fulfill the object. Actions are conscious, and different actions may be undertaken to meet the same goal. Actions are implemented through automatic operations. Activity Theory holds that the constituents of activity are not fixed, but can dynamically change as conditions change. It emphasizes that internal activities cannot be understood if they are analyzed separately from external activities, because they transform into each other.
Internalization is the transformation of external activities into internal ones.
Externalization transforms internal activities into external ones. It is also important when a collaboration between several people requires their activities to be performed externally in order to be coordinated. Mediation Activity Theory emphasizes that human activity is mediated by tools in a broad sense. Tools are created and transformed during the development of the activity itself and carry with them a particular culture - historical remains from their development. So, the use of tools is an accumulation and transmission of social knowledge.