Apex Messages in Salesforce are used for displaying messages such as a warning or an error in visualforce page. In this article I’ll demonstrate how to use different types of messaging options in VisualForce page.
Apex message severities are:

• CONFIRM
• ERROR
• FATAL
• INFO
• WARNING

CONFIRM:

Visualforce Page:

<apex:page controller="SuccessMessage"> 
<apex:pagemessages> 
</apex:pagemessages> 
</apex:page> 

Apex Class:

public class SuccessMessage
{ 
   public SuccessMessage()
   {
      ApexPages.addMessage(new ApexPages.Message(ApexPages.severity.Confirm, 'This is a success message'));
   }
} 

ERROR:

Visulaforce Page:

<apex:page controller="ErrorMessages"> 
<apex:pagemessages> 
</apex:pagemessages> 
</apex:page> 

Apex Class:

public class ErrorMessages
{ 
   public ErrorMessages()
   {
     ApexPages.addMessage(new ApexPages.Message(ApexPages.severity.Error, 'This is an error message'));
   }
}

WARNING:

Visualforce Page:

<apex:page controller="WarningMessage"> 
<apex:pagemessages> 
</apex:pagemessages> 
</apex:page>

Apex Class:

public class WarningMessage
{ 
   public WarningMessage()
   {
      ApexPages.addMessage(new ApexPages.Message(ApexPages.severity.Warning, 'This is a warning message'));
   }
} 

INFO:

Visualforce Page:

<apex:page controller="InfoMessage"> 
<apex:pagemessages> 
</apex:pagemessages> 
</apex:page>

Apex Class:

public class InfoMessage
{ 
   public InfoMessage()
   {
      ApexPages.addMessage(new ApexPages.Message(ApexPages.severity.Info, 'This is an informational message'));
   }
}