Facet Usage:

Facet Usage:
A facet consists of content in an area of a Visualforce component that provides contextual information about the data that is presented in the component. For example,<apex:dataTable> supports facets for the header, footer, and caption of a table, while <apex:column> only supports facets for the header and footer of the column. The<apex:facet> component allows you to override the default facet on a Visualforce component with your own content. Facets only allow a single child within the start and close tags.

<apex:page standardController="Account">
    <apex:pageBlock>
        <apex:dataTable value="{!account}" var="a">
            <apex:facet name="caption"><h1>This is 
              {!account.name}</h1></apex:facet>
            <apex:facet name="footer"><p>Information 
              Accurate as of {!NOW()}</p></apex:facet>
            <apex:column>
                <apex:facet name="header">Name</apex:facet>
                <apex:outputText value="{!a.name}"/>
            </apex:column>
            
            <apex:column>
                <apex:facet  name="header">Owner</apex:facet>
                <apex:outputText value="{!a.owner.name}"/>
            </apex:column>
        </apex:dataTable>
    </apex:pageBlock>        
</apex:page>