How is webform node type is defined in webform module?
The initial webform content type is created during install as an entity of type node. The code can be found in the webform_install() function:

$webform_type = array(
  'type' =&gt; 'webform',
  'name' =&gt; st('Webform'),
  'base' =&gt; 'node_content',
  'description' =&gt; st('Create a new form or questionnaire accessible to users. Submission results and statistics are recorded and accessible to privileged users.'),
  'custom' =&gt; TRUE,
  'modified' =&gt; TRUE,
  'locked' =&gt; FALSE,
);
$webform_type = node_type_set_defaults($webform_type);
node_type_save($webform_type);
if (variable_get('webform_install_add_body_field', FALSE)) {
  node_add_body_field($webform_type);
}


Additionally it might be worth noting, that you can turn any content type into a webform by checking the appropriate setting on the content types main configuration page. You can think of webform as an add-on extension to other content types. To find out which content types are "webform enabled" you should use the provided API functions. In webform-7.x-4.0-beta for example that would be webform_node_types().