What's the ThingPackage in metadata.xml ?

Hi, 

I'm writing a Java extension and editing the metadata.xml,  but I don't understand what the term ThingPackage stands for .  

According to the XML structure , the ThingTemplate must refer to ThingPackage which defines the name of the classname, while ThingShape can define classname directly.  

What's the relationship among ThingPackage, ThingTemplate ,ThingShape and Thing ? 

<ThingPackages>
 <ThingPackage className="" description="" name=""/>
</ThingPackages>
<ThingTemplates>
 <ThingTemplate aspect.isEditableExtensionObject="false"
 description=""
 name="" thingPackage=""/>
</ThingTemplates>
<ThingShapes>
 <ThingShape aspect.isEditableExtensionObject="false" className="" description="" name=""/>
</ThingShape

Regards,

Sean