List of the ApexPages.Severity

List of the ApexPages.Severity enum values are:

• CONFIRM
• ERROR
• FATAL
• INFO
• WARNING

For more help on this topic check out the documentation.

in the Apex Class can be used like this:

ApexPages.Message myMsg = new ApexPages.Message(ApexPages.severity, summary);
ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.FATAL, 'my error message');