iCal (or iCalendar) is an Internet file format (sometimes called a feed) for exchanging calendar information (such as events) between people. By converting your Drupal events into an iCal feed, you can export your events to others, for example, so people can import your events into Google Calendar.
Likewise, by reading an iCal feed from another source, such as from Google Calendar, you can export events from Google Calendar and import the events into Drupal.
Enable the iCal module
The first step toward either importing or export iCal feeds, is to install the Calendar module, and enable it together with the Calendar iCal module. Other modules are required, as indicated on the Administrator's Module page (ie. Views).
Exporting: creating your own iCal feed
When the Calendar and iCal modules are enabled...
Go to Views (Admin | Site building | Views), where you will see a new "Default Node view: calendar (Calendar)". It is disabled by default and greyed out. Click the Enable link to enable it.
I recommend that you do not Edit the new Calendar view (think of it as your master copy). So click "Clone" to make a copy, give the cloned calendar view a relevant name (eg. icalview). Then click Next to go your new view edit screen.
You will see a list of pre-defined views (under Default). Click iCal feed to begin to create it, and away you go!
Importing an iCal feed (eg. from Google Calendar)
You can import iCal feeds into Drupal in several ways:
1. Importing iCal directly into a Calendar view
I think this method has been superseded by the next option, but I'm leaving this section until it is confirmed.
3. Importing iCal into CCK fields
In addition to enabling the Calendar and iCal modules, you must also enable the the Date module, and its Date Tools and Date Repeat API modules. (iCal used to require Date Copy, but this is now part of Date Tools).
In Admin, in Content management select the new "Date Tools" option. Then select the Date Wizard. iCal is mentioned in the introduction.
There is a little more information in Advanced Help | Date API | Date Copy (misnamed! It is now part of Date Tools)