This article provides a step-by-step tutorial for creating a custom, multistep registration form via the Ctools Form Wizard in Drupal 7. If you'd prefer to solely use the core Form API, take a look at Building a Multistep Registration Form in Drupal 7, a previous blog post. In the interest of saving time, I'm going to be lifting some text directly from that post, given that there are a number of overlapping tasks.
Why use the Chaos Tools module to build a multistep form? Well, Ctools offers a number of tools that build upon the core Form API, allowing you to create a multistep form faster. This includes providing a method for caching data in between steps, adding 'next' and 'back' buttons with associated callbacks, generating a form breadcrumb, etc.