Edit: you can also use ctools

Is there too much text on your page? Do you need to allow users to un-clutter an interface, or hide some of the interface until your users need it? Don't go and grab any extra JavaScript, Drupal can do collapsible fieldsets out of the box!

However, there are some steps involved:

The "disabled" attribute for form fields is a great little user interface helper that you can use to help protect data from accidental changes. It locks down an HTML input form element so that it can be viewed, but not be changed. This is handy when displaying a form to a user where you need an extra layer of protection for the data.

What happens when the user goes to submit a form with disabled elements? The value of the disabled elements are not passed back to the browser - this can quickly turn into a data validation and/or submission issue.