The extensionURL tag is used to reference resources deployed in the web area that are visible to the browser.  It figures out the extension name based on the extension defined in definition for the entry being processed. So this is the code from the VideoEntry extension that David wrote:
 <object height="290" width="400"
  type="application/x-shockwave-flash" data="<ssf:extensionUrl url='swf/FlowPlayerDark.swf'/>">
     <param name="flashVars" value="config={'autoPlay':false,'initialScale':'scale','videoFile':'<ssf:fileUrl entity='${ssDefinitionEntry}'/>'}"/>
     <img alt="Sorry, Adobe Flash Player is required to view videos" src="<ssf:extensionUrl url='img/no-flash.png'/>" />
 </object>

Referenced jsps should be relative to the current jsp.  The directory structure that the extension uses is maintained when deployed
 
You can have as many definitions as you want inside an extension.