What is an Office 365 shared mailbox?

Microsoft calls their shared inboxes “shared mailboxes.” It's an email inbox that can be accessed by multiple teammates.

An Office 365 shared mailbox can be created by an account administrator and accessed by teammates that the administrator has invited. (For example, sales@pawnee.com.) Every person assigned to the shared mailbox has full access to it, with the ability to read incoming emails, reply, forward messages, and create new emails. Everyone can also see which emails have been replied to and which haven't, and they can read replies that other members have sent. All emails sent from the shared mailbox display the shared mailbox address, such as contact@pawnee.com — not the individual user account addresses. 

Office 365 shared mailboxes don't have separate logins. Users invited to the mailbox will access it through their individual email accounts. For this reason, any shared mailbox member must have an Office 365 subscription.

You can create as many shared mailboxes as you like, so each department or function of the business (sales, PR, customer service, etc.) can have a separate mailbox. 

It's important to note that you need to subscribe to Office 365 Business Premium to create a shared mailbox. You'll also want to know that there is a 50MB storage limit. To gain more storage, you’ll need to assign a license to the mailbox.