Some additional information when you think about Exchange Calendar overlay(s).
You might get the feeling that we talk in this article that more than one Exchange Calendars are possible. http://office.microsoft.com/en-us/sharepoint-foundation-help/work-with-multiple-exchange-and-sharepoint-calendars-in-one-sharepoint-calendar-HA101777171.aspx
When you use a SharePoint Calendar and you chose to add an Exchange Calendar as an overlay it will add always the Exchange/Outlook calendar information for your own account and you cannot chose a particular Calendar e.g. for a meeting room.
Calendar Name: Any text you want to use.
Outlook Web Access URL: This link will be shown on the left pane with Pink as background color. The link can be anything but it make sense to use the link to OWA. For more information please contact your Exchange administrator.
Exchange Web Service URL: It is important to use the right URL pointing to the Exchange WebService from where SharePoint gets the information. For more information please contact your Exchange administrator.
With all these fields to enter information you can configure that each user will see information from the own information in the Exchange Calendar.
SharePoint knows the user who logged in to the SharePoint Calendar page. With that user information SharePoint is able to contact the Exchange WebService to retrieve the calendar data for this particular user.