The Mega Drive II is basically just a re-designed Mega Drive.
The Mega Drive II was much smaller and squarer then the original Mega Drive. It did not have the headphone jack, volume control or RF TV output. The A/V port changed to a custom multi out port that now provided stereo sound to T.V's (previously only mono was output by the Mega Drive, the headphone jack had to be used for stereo sound). The power lead port was made smaller and a different AC adapter was used. Finaly the power switch was changed to a push button.
Both the original Mega Drive and the Mega Drive II could be connected to a Sega CD and 32X add-ons. The Mega CD was even remodeled to better fit the Mega Drive II. A specific version of the Mega Adapter/Master System Converter was also designed.
Adapted from Miles Mason's Genesis 2/Megadrive II text