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.
This game featured the introduction of an EASY modifier, the introduction of a time bar (showing how far in the song one is) and the debut of Ryutaro Nakahara and Kosuke Saito.
beatmania IIDX 4th Style for PS2 was a direct port of the current arcade version, plus preview songs from 5th Style, as well as new songs, and two revivals from previous versions.
Source: Wikipedia, "Beatmania IIDX 4th Style", available under the CC-BY-SA License.