Retro/Grade is a rhythm game. Yes, you're controlling a little space ship, but the whole time, you're reversing through the levels, taking your shots back after the game was completed previously by the protagionist.
It's music style is electronic, all done in house, and it is excellent. Who's The Boss is the standout track in my opinion, but it's all good. Taking the shots back happens on the beat, all while avoiding the enemie's fire, which is also being taken back in time to the beat.
It's closest in style to Frequency or Amplitude, with the style of music and the many bright colours distracting you from the beat. The higher the difficulty selected, the more bars are active, starting with two and working up to 5. It pulls some inspiration from Rock Band, even letting you hook up an USB guitar to strum along to the game, but as I mentioned before, much closer to Amplitude/Frequency in visual style and music.
All and all, an excellent rhythm game with a great soundtrack and plenty to do: 10 mission campaign mode, 6 difficulties for that, a large challenge mode, a music mix mode, and lots of little extras.