Hi Guys,
Following going through the salesforce docs,I came across the below code snipet. On change event calls java script function rerenderStates ,which is nonewhere defined on the page ,is that created by default? Also Param has blank value ,how's country passed to param?The code is working fine.Throgh param country  value selected is passed to country variable in controller and accordingly the dependent picklist show the values.
<apex:page controller="CountryStatePicker">
   <apex:form >
      <apex:actionFunction name="rerenderStates" rerender="statesSelectList" >
          <apex:param name="firstParam" assignTo="{!country}" value="" />
      </apex:actionFunction>

   <table>
      <tr>
        <th>Country</th>
          <td>
             <apex:selectList id="country" styleclass="std" size="1" 
                value="{!country}" onChange="rerenderStates(this.value)">
                    <apex:selectOptions value="{!countriesSelectList}"/>
             </apex:selectList>
          </td>
      </tr>
      <tr id="state_input">
        <th>State/Province</th>
          <td>
            <apex:selectList id="statesSelectList" styleclass="std" size="1" 
                 value="{!state}">
                   <apex:selectOptions value="{!statesSelectList}"/>
            </apex:selectList>
          </td>
      </tr>
   </table>
   </apex:form>
</apex:page>


Thanks in advance!