The general widget definition for BE form fields in Magento is:

<?xml version="1.0"?>
<widgets>
    <widget_identifier type="blochtype/like_in_normal_blocks" translate="name description" module="modulname">
        <name>Widgetname</name>
        <description>A useful description</description>
        <is_email_compatible>1</is_email_compatible>
        <parameters>
            <!-- field definition goes here -->
        </parameters>
    </widget_identifier>
</widgets>

The following snippets show how to define different form fields for the widget form:

• Template selector:

  <template translate="label">
      <label>Template</label>
      <visible>1</visible>
      <type>select</type>
      <value>cms/widget/link/link_block.phtml</value>
      <values>
          <default translate="label">
              <value>cms/widget/link/link_block.phtml</value>
              <label>CMS Page Link Block Template</label>
          </default>
          <link_inline translate="label">
              <value>cms/widget/link/link_inline.phtml</value>
              <label>CMS Page Link Inline Template</label>
          </link_inline>
      </values>
  </template>

• Fixed template:

  <template translate="label">
      <label>Template</label>
      <required>1</required>>
      <visible>0</visible>
      <type>text</type>
      <value>asksheldon_teaser/link.phtml</value>
  </template>

• Text input:

  <column_count translate="label description">
      <visible>1</visible>
      <required>1</required>
      <label>Columncount</label>
      <description>Number of Colums</description>
      <type>text</type>
      <sort_order>1</sort_order>
  </column_count>
  

• Enable / disable select:

  <enabled translate="label">
      <label>Enable</label>
      <frontend_type>select</frontend_type>
      <source_model>adminhtml/system_config_source_yesno</source_model>
      <sort_order>10</sort_order>
      <show_in_default>1</show_in_default>
      <show_in_website>1</show_in_website>
      <show_in_store>0</show_in_store>
  </enabled>

• Multiselect (customer groups):
  

  <customer_groups translate="label comment">
      <label>Customer groups</label>
      <frontend_type>multiselect</frontend_type>
      <source_model>adminhtml/system_config_source_customer_group</source_model>
      <sort_order>11</sort_order>
      <show_in_default>1</show_in_default>
      <show_in_website>1</show_in_website>
      <show_in_store>0</show_in_store>
      <comment>Groups for Friends & Family discounts.</comment>
  </customer_groups>

• Category Selector:

  <catid translate="label">
      <visible>1</visible>
      <required>1</required>
      <label>Category</label>
      <type>label</type>
      <helper_block>
          <type>adminhtml/catalog_category_widget_chooser</type>
          <data>
              <button translate="open">
                  <open>Select Category...</open>
              </button>
          </data>
      </helper_block>
  </catid>

  To determine the category ID and load the category in the widget block class, you can use this snippet:

  $iCategoryId = str_replace('category/', '', $this->getData('catid'));
  $oCategory   = Mage::getModel('catalog/category')->load($iCategoryId);