Basically Rock Band in a portable format except no plastic instruments or singing into a mic, you need to constantly switch between instruments (tracks) a la Harmonix's earliest rhythm games Frequency and Amplitude, and NO MULTIPLAYER.
The way the instrument switching works is you play a particular instrument and if you successfully complete a "phrase" that instrument will play on its own for a while so you then switch to another track, play that phrase, and repeat. There's an "Extras" mode where you can play just the instrument(s) you want but on the World Tour mode, which is just like the World Tour mode on the non-portable systems, instrument switching is mandatory.
I'm mediocre at best at rhythm games but I found the difficulty levels well-suited for my skills. Easy is almost snooze inducing. Hard is doable for most of the songs and Expert I fail a lot at.