--updated added code also--
you can define template for your form in template.php

After you have above code in your template.php then create a user-register.tpl.php in your theme folder and simply place code below:-

These snippets allow you to override the default user registration layout using a custom user_register.tpl.php.

If you want to customize the full page layout, click through to the customizing the login, registration and request password full page layout handbook page.

step 1 of 2

In a text editor like notepad.exe, create a file called template.php using the the following snippet. If you already have a template.php file, simply add it to your existing one (remembering to omit the opening and closing PHP tags if you're adding it to an existing template.php file, i.e. <?php and ?).

step 2 of 2

1. In a text editor create a new text file and paste the following snippet into it. Save the file with the filename user_register.tpl.php
2. Edit the style sheet classes and content to suit
3. Upload your edited user_register.tpl.php to your active theme folder

For use with Drupal 4.7.x

Extra instructions or content can go here, just below the registration form

Style sheet reference

For controlling how your registration form looks using your style sheet, this is what the rendered registration form HTML and class names are by default: