What is the Difference between Ajax and ActionPoller in Salesforce?

These are the AJAX action tags that support the calling of action and refresh the field only not the entire Visualforce page.1. <apex:actionFunction> - Provides support for invoking controller action methods directly from JavaScript code using an AJAX request. An <apex:actionFunction> component must be a child of an <apex:form> component.

2. <apex:actionPoller> -  A timer that sends an AJAX update request to the server according to a time interval that you specify. Update request can then result in a full or partial page update. You should avoid using this component with enhanced lists.

3. <apex:actionStatus> -  A component that displays the status of an AJAX update request. An AJAX request can either be in progress or complete