Forms allow your user to interact with your website.  They allow you to collect data from the user.  You see them in contact forms, shopping carts, and any other time the website needs to interact with a user.

The form is only part of the solution, and our tutorial goes into that.

http://htmldog.com/guides/html/beginner/forms/

This tutorial is only the tip of the iceberg, there are plenty of  other html tags, but this is a good place to start.