The configure item under a SQL Servers alerts tab displays a list of all the alerts that can be triggered.
The default list can be added to by creating a custom SQL alert
Click the alert's name to open it's properties window:
If the alert uses a threshold, like the number of items in the disk queue, then a major and minor threshold can be configured here.
Enabling sending of emails when the alert is triggered and also writing to the Windows event log is set here.