Each form has it's own amount of flexibility in customization.
There are four variations of the small Flash form, but each comes with pre-generated HTML code and little customization.
The large Flash form has customizable field selection, color scheme, and header text. The HTML code for this form is generated based on the customizations you assign to this form.
The HTML form is the most basic with the ability to customize the fields collected in the form. However, the code itself that is generated can be manipulated to fit style properties of your choice.