Technically, a user could define nearly anything to be "an event." In practice, events fall into the following two categories:
The concept of notices is used to provide "reminders" at specified times. A common notice is to inform workers that "Timesheets are due by noon today."
Notices are differentiated from actual events by one major factor:
Place. An event must occur at a defined place. Note, however, that a defined place need not be a physical space. For example, a cyberspace chat scheduled from 6pm to 8pm is an event, and it does have a place - online - even though a physical room reservation is not required. A notice is not associated with a place, physical or virtual.