< hidden >
You may want to submit data without showing them on the form. This may be usefull if you create a set of sequential forms. Each letting you fill out part of the data. The data of the previous forms can be put in hidden tags. If you wouldn't use these the data would be lost. You could also temporarily save these data at the server, but that requires a lot more programming. E-commerce applications mostly do that, preventing data to be sent back and forth, removing possible security problems.
<INPUT TYPE="hidden" NAME="name" VALUE="value">
This code will not appear on screen. Check the source of this page if you don't believe me. The code is there. Notice the VALUE attribute. This contains the actual data, related to the variable specified in the NAME attribute. Since there is no box to fill out, this is the only way for this tag to contain data.