Events:

Within the delegation model, an event is an object which describes a state modify in a source. It can be produced as a consequence of a person interacting along with the elements in a graphical user interface. A few of the activities which causes events to be produced are pressing a button, entering a character through the keyboard, choosing an item in a list, and clicking the mouse. Several other user operations could also be cited as instances.

Events  might  also  occur  which  are  not  directly  caused  through  interactions along with  a  user interface. For instance, an event might be produced whenever a timer expires, a counter exceeds a value, software or hardware failure occurs, or an operation is finished. You are free to declare events which are suitable for your application.