Retro Emulator is an emulator for Master System and Sega Genesis (Megadrive) that lets you play all the games from both consoles, right on your Android.
As with other emulators, Retro Emulator doesn't come with any roms. This means if you want to play a game on your device, you have to download the roms from another website first ... which is relatively simple. Once you've downloaded the roms (and you've unzipped them if they came in a compressed file), you can start playing.
Retro Emulator's main downfall is that it comes with very few configuration options. On the bright side, you can start playing instantly (once you have a rom), but unfortunately, you won't find many options to customize your experience. Also, the controls aren't very precise at times.
Retro Emulator is a decent emulator, but it's nothing special. In fact, there are better alternatives for Android when it comes to emulating both Master System and Sega Genesis/Megadrive.