When you are invited to a meeting, you will receive an invitation by email.  The event will also appear on your calendar and will be marked tentative.  Email invitations are identifiable by the calendar icon as show below.

To reply to a meeting:

1. Double-click the invitation to open it.
2. Choose one of the options from the Respond group in the ribbon.
   
3. When you respond to a meeting, you will receive a dialog box asking if you want to
   -Edit the response before sending (include a message with your reply)
   -Send the response now (notify the meeting organizer whether you accept or decline)
   -Don't send a response (does not notify the organizer of your response)
4. Select a response and then click OK
   

   

IMPORTANT: Outlook's calendar is closely integrated with email.  When replying to a meeting, always choose to either Edit the response before sending OR send the response now.  Do not use the Don't Send a Response option.  If you select this option, the organizer will not see whether or not you have accepted or declined the meeting.  Your attendance status will only appear on your calendar.

Once you response to a meeting, the invitation will disappear from your mailbox.  If you accept the meeting, it will remain on your calendar.  If you decline the meeting, it will not appear on your calendar.  Never delete an invitation without replying to; this will also delete it from your calendar and you will not be able to retrieve the meeting invitation.