The article considers a formal model describing human activity in the information aspect. Like any process description methodology, a certain basic abstraction is used. The model is based on the previously introduced abstraction of the mechanism of action. The mechanism of action includes the Elements on which the action is performed, the Function that is performed on the elements, and the Instrument with which the function is performed. When the action is performed, the Result is formed. The activity model is formed as an interconnected set of mechanisms describing the actions performed by a person in the realization of business processes of an enterprise. The article describes the structure of the model, the method of its formation and an example of its use to describe the user activities in an information system. The proposed activity model can be used to solve problems of formal analysis and optimization of business processes, as well as for the formal description of other types of human activity.