The full documentation is at http://drupal.org/documentation/modules/webform.
You can achieve the conditional field with Webform Conditional.
This modules enhances the existing Conditional Rules of the Webform Module. It adds the ability to make components dependent on select components(i.e. listboxes, radio buttons, and checkboxes) that appear on the same page.
Here's a youtube video Screencast demo.
Webform already has support for conditional fields on multi-page forms. For example Field C on Page 2 can be dependent on Field A on Page 1. This module adds the ability to have conditional fields on the same page. This module adds the ability to have Field B on page 1 be dependent on Field A also on Page 1.
Check the version of the Webform module; you may have it already.
The conditional field module will let you set conditions for a field to appear or not.
For example if the question is do you have a cat? Then fields like:
Siamese Stray Kitten
And if you click on the radio button dog:
German Shepperd Bulldog Pitbll
http://drupal.org/node/1704126 does this