Windows Defender came out on Vista, and for Vista it wasn't very good, since Vista wasn't very good. They changed the name to Security Essentials on Windows 7, although 7 shipped with a copy of Vista's defender installed as well, and in MS infinite wisdom changed it back to Windows Defender on 8.
If you open Security Essentials on Windows 7, or Defender on 8, or Endpoint Protection, they all look the same, because they are the same, with two exception, being Endpoint Protection is used in corporate environments and sends back data to a central managed server for reporting and data collection and Defender 8 not having a notification icon shown unless it detects something.
Excluding Vista's first attempt at windows defender, they run the same definitions.