How to Use apex:pageMessage in Visualforce page?

<apex:pageMessage> 

              This component should be used for presenting custom messages in the page using the Salesforce pattern for errors, warnings and other types of messages for a given severity. See also the pageMessages component.

This tag supports following attributes:

Attribute	Description
detail	The detailed description of the information.
escape	A Boolean value that specifies whether sensitive HTML and XML characters should be escaped in the HTML output generated by this component. If you do not specify escape="false", the character escape sequence displays as written. Be aware that setting this value to "false" may be a security risk because it allows arbitrary content, including JavaScript, that could be used in a malicious manner.
id	An identifier that allows the component to be referenced by other components in the page.
rendered	A Boolean value that specifies whether the component is rendered on the page. If not specified, this value defaults to true.
severity	The severity of the message. Values supported are: 'confirm', 'info', 'warning', 'error'
strength	The strength of the message. This controls the visibility and size of the icon displayed next to the message. Use 0 for no image, or 1-3 (highest strength, largest icon).
summary	The summary message.
title	The title text for the message.

Example:

<apex:page standardController="Opportunity" recordSetVar="opportunities"

    tabStyle="Opportunity" sidebar="false">

    <p>Enter an alphabetic character for the "Close Date," 

       then click Save to see what happens.</p>

    <apex:form >

        <apex:pageBlock >

        <apex:pageMessage summary="This pageMessage will always display. Validation error 

           messages appear in the pageMessages component." severity="warning" strength="3" />

        <apex:pageMessages />

        <apex:pageBlockButtons >

            <apex:commandButton value="Save" action="{!save}"/>

        </apex:pageBlockButtons>

            <apex:pageBlockTable value="{!opportunities}" var="opp">

                <apex:column value="{!opp.name}"/>

                <apex:column headerValue="Close Date">

                    <apex:inputField value="{!opp.closeDate}"/>

                </apex:column>

            </apex:pageBlockTable>

        </apex:pageBlock>

    </apex:form>

</apex:page>

The above code generated the below page: