Notifications are one of the tools you can use to keep track of your systems. They are automatic messages that are sent to specific users when their conditions are met - such as when a critical incident occurs.
Each user has their own notification profile, which specifies what level of notifications are sent - whether it's everything that happens to an asset, all incidents, or just critical incidents. It also specifies where these notifications are sent - the user interface, a specified email address, or a specified pager. You can set different levels for each destination, so that critical incidents go to the pager but less crucial issues go to the email address. If you have the user admin role you can configure a notification profile using this procedure.
The Command Line Interface for Ops Center lets you view and delete notifications as well, although configuring a notification profile can only be done through the UI.