13) How to display error messages in the Visualforce page?

To display error messages in the Visualforce page add below tag where you want to display the error message.

Visualforce page:
<apex:pageMessages />

Apex Controller:
 ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.ERROR,'Error Message.');
ApexPages.addMessage(myMsg);

(OR)

ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR,'Error Message.'));