Colored bars fall down on the screen (white and blue for the keys, red for the scratch) and must be validated by hitting or scratching when they exactly reach the bottom of the screen. Each key is matched to a sound that adds to the musical background, giving the player the feeling of actually playing the music. This principle has been carried on to pop'n music (see the article for more)
As for DDR, success depends on the status of the gauge, but it has to be up to 80% at the end of the song to considerate it cleared (and this applies even if the gauge has been emptied at some point).
Beginners will have a hard time getting accustomed to beatmania IIDX though, it's not easy to hit the keys properly without mistaking them when playing for the first time. Moreover, notes are not located on "full beats", therefore even easiest songs, even in BEGINNER level, might seem like mayhem for uninitiated people. It all comes up with time to get used to it... then, as you progress and get the key positions right, as well as the rhythm patterns, you'll start clearing some songs. Don't despair at first! You have problem at low levels? It'll pass! And don't get it wrong, I've been through this phase as well...
Also notice that, unlike what could be thought, IIDX doesn't only feature boombass techno-trance beats. On the contrary, a whole panel of genres is in: electro, rap, jazz, rock, (neo-)classical... there definitely are some tunes that some aficionados of a particular genre will appreciate, without even knowing it comes from beatmania IIDX.
Last thing: BEATMANIA AIN'T NO GUITAR HERO RIPOFF! Hello? Hello!? Anybody home, hunh? Think, little noob, think! How come beatmania, dating back from 1997, could have in any way copied Guitar Hero whose first tome was issued by the end of 2005? Shortly: study your videogaming history before saying ignominies.