AHAH, is a method for updating webpages dynamically using Javascript, similar to Ajax, but with the difference that the response from the request is used directly without parsing on the clientside. This means that server responses need to be text or already include valid XHTML/HTML structure.

Basically when a user performs an action (like clicking a link or form button) AHAH can respond to this action by calling a URL and returning the URLs respond(HTML or text) without needing to refresh the whole page.

The upload module uses AHAH to allow uploading of images and files without needing to refresh the other page.