Forms are used on websites to collect user information through various form elements like text fields, checkboxes, radio buttons, menus, and buttons. Forms have a front-end visible to users and a back-end program that processes the submitted information. Common XHTML tags like <form>, <input>, <select>, <option>, and <textarea> are used to structure forms and create different form elements. Form layout and design should keep the form short, simple, and well organized to make it easy for users to complete.