In HTML, a form is a collection of input elements whose contents get submitted to a CGI program by the web browser.
Typically, a form starts with <form> tag and ends with </form>. The start tag has two mandatory parameters: the "action" parameter that says where the CGI program is, and "method" parameter that says which HTTP access method should be used: GET or POST.
The form then contains normal HTML. The elements inside are created with <input> and <textarea> elements.
Here's an example of a simple form:
<form method="post" action="/cgi-bin/subscribe.pl">
<p>Enter your E-mail address to subscribe to the mailing list:
<input type="text" name="email" size="40"
maxlength="100" />
<input type="submit" value="Subscribe" />
</p>
</form>
This will create a simple form that has one text input box.
Everything2 uses forms very heavily. Forms containing a textarea are used to enter writeups, and search box, chatterbox and the chatterbox room thing are made with forms. Many superdocs also have forms.