An area of a page that includes content from a second template page. Template pages are
Visualforce pages that include one or more <apex:insert> components. The <apex:composition>
component names the associated template, and provides body for the template’s <apex:insert>
components with matching <apex:define> components. Any content outside of an
<apex:composition> component is not rendered.

A placeholder for content that is rendered in a specific part of the parent component, such as the
header or footer of an <apex:dataTable>.
An <apex:facet> component can only exist in the body of a parent component if the parent
supports facets. The name of the facet component must match one of the pre-defined facet names
on the parent component. This name determines where the content of the facet component is
rendered. Consequently, the order in which a facet component is defined within the body of a
parent component does not affect the appearence of the parent component.
See <apex:dataTable> for an example of facets.

Note: Although you can’t represent an <apex:facet> directly in Apex, you can specify it on a
dynamic component that has the facet. For example:

A set of content that is grouped together, rendered with an HTML <span> tag, <div> tag, or
neither. Use an <apex:outputPanel> to group components together for AJAX refreshes.

The email publisher lets support agents who use Case Feed compose and send email messages
to customers. You can customize this publisher to support email templates and attachments. This
component can only be used in organizations that have Case Feed and Email-to-Case enabled.
Ext JS versions less than 3 should not be included on pages that use this component.

An area of a Visualforce page that demarcates which components should be processed by the
Force.com server when an AJAX request is generated. Only the components in the body of the
<apex:actionRegion> are processed by the server, thereby increasing the performance of the
page.