The standard for J1939 describes an industrial protocol based on CAN. It originated as a vehicle bus in the American car and heavy-duty truck industry but has since found use in other types of applications.
NMEA2000 is an extension to the base J1939, aimed specifically at marine applications. It contains all the standard J1939 specifics and additional marine specific messages. NMEA2000 devices and J1939 devices work on the same network.
To use the NMEA2000 name, a device must pass a certification process.