a.k.a. feedback form -or- interactive form -or- CGI form -or- script -or- order form
A Web page that accepts user input. With an online form, you can usually input comments, order products, sign up for newsletters, or search for information. At the bottom of a form, you will usually see submit and reset buttons; these allow users to either "send in" the information or clear out the form and start over. For a more technical discussion of how it works, see the CGI definition.
Here's an example of when you'd use a form: You see the opportunity to sign up for an e-mail newsletter service on a particular Web page, and in order to receive the newsletter, you must fill out a CGI form. Once you hit submit, the form is sent to the server, which processes the information and adds you to its database of newsletter recipients.