From https://www.salesforce.com/us/developer/docs/pages/Content/pages_compref_insert.htm

apex_insert.html

<!-- Page: composition -->
<!-- This page acts as the template. Create it first, then the page below.  -->
<apex:page>
  <apex:outputText value="(template) This is before the header"/><br/>
  <apex:insert name="header"/><br/>
  <apex:outputText value="(template) This is between the header and body"/><br/>
  <apex:insert name="body"/> </apex:page>
  
<!-- Page: page -->
<apex:page>
  <apex:composition template="composition">
  <apex:define name="header">(page) This is the header of mypage</apex:define>
  <apex:define name="body">(page) This is the body of mypage</apex:define>
  </apex:composition>
</apex:page>
  
The example above renders the following HTML:
  (template) This is before the header<br/>
  (page) This is the header of mypage<br/>
  (template) This is between the header and body<br/>
  (page) This is the body of mypage