A calendar on a SharePoint site.
Why use a calendar on a SharePoint site? Because it's a great way to share information using the familiar context of a calendar. You might use a calendar on a SharePoint site to keep track of project deadlines, travel and vacation schedules, or even coworkers' birthdays.
You can think of a calendar on a SharePoint site as similar to a whiteboard calendar that you might put up in your building's hallway or break room. Anyone who can get to the calendar can contribute. Like a whiteboard in a hallway, the SharePoint calendar is in a public place: the SharePoint site. Anyone who has access to the site will be able to see and use the calendar.