Activity Diagram is one diagram used to model a system in the Unified Modelling Language. Activity Diagram is used to draw the activity existing in a system, connecting, splitting, or merging the sequence of tasks involved.
A simple example of an Activity Diagram of an activity in a library (activity of borrowing a book) is as follows. The activity is started by a member bringing his/her member card and gives it to a librarian. The librarian then checks the member card, and checks the book to be borrowed. If the book is available, then the librarian gives the book to the member, then the activity stops. If the book is unavailable, then the activity stops.
In some cases of activities, a number of additional steps can occur within the activity including splitting direction of tasks into two or more paths, or merging two or more paths into a consequence task. In such a case, the following symbols need to be referred, so that all existing steps can be drawn into a comprehensive diagram.