Ajax.module will AJAXify any Drupal form. This essentially allows for validation, preview, and submission without a page refresh. Ajax.module supports most forms out of the box. Ajax.module provides both an API for module maintainers as well as an admin interface for easy configuration.
By default in Drupal, when a form is submitted, the values of that form are sent through a series of Drupal validation procedures. When these validations are completed the validation messages are normally sent back to the user with a standard page refresh. For example, if a user fills out a form which requires a first name, but the user leaves these fields blank, they will be presented with a message that says "First Name is Required".
This module allows the validation messages to be sent back to the user without a page refresh, so they appear in realtime. This makes for a more streamlined user experience. If the submitted form has no errors, the page will be redirected to the appropriate submission landing page.