Hidden controls are not displayed and cannot be manipulated by the user, nor can their value be changed. They are used to supply a fixed name/value pair along with the rest of the form. This could be useful if the program that processes the submission does a number of things, but you want the specific form to do only a subset of these. Hidden controls have a TYPE of hidden. For instance, a program that subscribes and unsubscribes people from a mailing list might have a field called action that could have the value subscribe or unsubscribe, and you want to make a form that only allows unsubscribtion. You could do this with something like:

<FORM METHOD="POST" ACTION="/cgi-bin/html/formdump.cgi" 
      ENCTYPE="multipart/form-data">
<P>To unsubscribe, enter your e-mail address:
<INPUT TYPE="text" NAME="email" SIZE="30">
<INPUT TYPE="submit" VALUE="Submit">
<INPUT TYPE="hidden" NAME="action" VALUE="unsubscribe">
</FORM>

<FORM ACTION="/cgi-bin/html/formdump.cgi" 
      METHOD="GET" ENCTYPE="application/x-www-form-urlencoded">
<P>Enter your e-mail address:
<INPUT TYPE="text" NAME="email" SIZE="30">
<P>Subscribe
<INPUT TYPE="radio" NAME="action" VALUE="subscribe">
Unsubscribe
<INPUT TYPE="radio" NAME="action" VALUE="unsubscribe" CHECKED>
<INPUT TYPE="submit">
</FORM>