In IIDX games, players recreate the musical score of songs available to them through the use of a DJ simulating controller. The player must coordinate their hands in order to hit the correct keyboard buttons, spin the turntable, or a combination of both in correspondence to color coded notes that approach the judgement bar displayed on screen.
Source: Wikipedia, "Beatmania IIDX", available under the CC-BY-SA License.
beatmania IIDX 3rd Style removed the 4-keys mode from the game and replaced it with a Light7 difficulty. This gave most songs a completely separate, easier notechart. In addition, 3rd Style introduced Free Mode and Extra Stage.
Source: Wikipedia, "Beatmania IIDX 3rd Style", available under the CC-BY-SA License.