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: