What is BMS?
(lifted from here )
Short Answer: Be-Music Source (BMS) is a file format for rhythm game charts.
In a rhythm game, the player presses buttons in accordance with the music. The song refers to the music itself that plays, while the chart for the song is the "gameplay" - i.e. the sequence of buttons you have to press when you play the song in game.
But a .bms chart is just a sequence of notes in a file. To play the charts, you use a BMS Player like Lunatic Rave 2 or beatoraja. Simply put, a BMS player is the game. It reads the chart files, and challenges you to clear the song.
Note that we also use BMS to refer to related formats like .bme, .bml, .bmson and .pms.
While BMS is technically a file format, we sometimes just refer to the game as "BMS" (or PMS for 9Keys).
BMS supports multiple play modes corresponding to the amount of keys, but in this thread we're gonna focus on the play modes replicating the ones from beatmania IIDX, namely 7KEYS and 14KEYS.
Where do I start?
wcko87 has written a comprehensive guide for setting up beatoraja - the most modern and usable BMS player out there as of posting.