Answer: Shared mailbox is a mailbox that is being used by multiple users and they all have the same permission to the mailbox.
Please note that that the Shared Mailbox account is disabled by default and it is never used to login to the mailbox.
To Create A Shared Mailbox In Exchange Server 2010 follow the steps below:
Open the Exchange Shell and type the following command:
New-mailbox servicedesk –shared –userprincipalname firstname.lastname@example.org
After creating the mailbox you use the Manage Full Access the Permission to give all the users that will Share the Mailbox permission to use it.
To do that, Right Click on the mailbox under Recipient Configuration.
Now you need to select the users that will Share the mailbox by clicking on Add and selecting the users.
After Selecting all users click on Finish.
Next tell the users to add the new mailbox to their outlook Client.
Update : Due to the popularity of this Post I decided to make a video on how to do it: