xen discuss - Jul 2009 - difference between "ethernet" and "bridge" interface types

Curious if someone could point me to any documentation which describes 
the difference between an interface type of "ethernet" vs.
"bridge".

I was trying to set up a domain with virsh to use the second nic on a 
server, in the process I noticed that the default vnic that was created 
during the install was specified as

    <interface type=''ethernet''>

When I looked at dumpxml after doing a virsh attach-interface, I noticed 
the new vnic that was added was specified as
   
    <interface type=''bridge''>

During some tinkering I discovered that they are not the same, for 
example, I cannot specify

    <interface type=''ethernet''>
      <source bridge=''igb1''/>

As the source bridge appears to be ignored. Instead, I *must* specify it as

    <interface type=''bridge''>
      <source bridge=''igb1''/>

  --joe