Whenever someone sends you a new private message, it goes into your message inbox. There is one common way to view and act upon your inbox - just click on the Inbox button on the upper right corner of the page.
At any time you can send a message to some of your co-workers or to a group of employees (on a respective position or a from a respective department).
Notifications section is where you can be constantly apprised of any changes in your account (e.g. new employees or locations are added) or any situation in your company that needs immediate attention (e.g. when there is a shortage of employees in any of the locations).
You can also check from here if there are any pending employee requests.
Announcements section is where you can see or post announcements which are used to inform employees about any company updates.
Notifications and Announcements are also visible on the Dashboard.