Figured I would post a couple screenshots from Theatrhythm on the 3ds. Not the best pics since I took them with my cell phone and was trying to play at the same time, but it will give you an idea 
In this mode you tap the screen whenever a music note touches the crystal to earn rhythmia
in this one you are walking and hitting the notes as you go along, you can get a chocobo in this mode and get an item from Mog and other characters
after you complete a song it shows you how many critical, good and bad hits you had
after you complete a song your characters get experience points to level up, and learn new spells
after you finish all the songs you get rhythmia which is used to unlock crystals
after this set of songs I’ve unlocked the Yellow Shard
Hope that gave you a slight idea on what this game is like. It’s really hard to describe but it’s a ton of fun especially if you are a Final Fantasy fan at all! I’ll keep trying to post better pictures as I go along!
I just beat the game over the weekend! Fantastic! The Chaos Shrine is really kicking my ass, though.
Awesome! I can’t wait to play more! Chaos Shrine is so intense, frustrating but so fulfilling when you beat a song!