Google Calendar allows users to create "events" for appointments, meetings, reminders, etc. Users can invite anyone with an email account to the event and set notifications to be reminded about the event via SMS message and/or email.
Events have many options. Users can create Hangout links for others to join a video call, documents can be attached to each event, and duration can be set to any time.
A unique feature of Calendars is the ability to create multiple calendars in order to share them with different groups of people but have all appear on one interface. For instance, you can have one calendar for the students in your class and another for your family events. Both calendars will show up on your Calendar page but the users in each calendar cannot see the events on a calendar other than the one they are shared with.
A built-in Calendar called "Reminders" can be switched to "Tasks" to enable the user to set a task with details and due date. This Task feature syncs with the Task feature in Gmail.
Google Help Center: Calendar