An ICS file is a universal calendar format supported by several email and calendar programs including Microsoft Outlook, Google Calendar, and Apple Calendar. It enables users to publish and share calendar information on the web and over email. ICS files are often used for sending meeting requests to other users, who can import the events into their calendars.
An ICS file can be added to a lot of calendars other than Google calendar. The following points are links to a few of those calendars but don't worry if you don't find your favorite calendar here, trying searching for it.