Pronounced "bom-bah ee play-na". The native music of Puerto Rico. They're actually two distinct music styles but are mentioned as one in general.
Bomba is chatacterized by the influence of African culture on the island. Three large drums are played in unison but with different rythms (often making them sound like there's many more). The one thing that makes it unique, however, is the fact that the drummers play according to the dancer's movements. That is, the dancer "asks" for the rythm with his/her movements. With the use of rapid steps, pauses, and even shaking his/her clothes, the dancer creates the wonderful music known as Bomba.
Plena, however, is different. It uses the three different drums called pandero, each of a different size. One sets the base beat for the music, another "follows" that rythm, and the third plays on the off-beat, alternating the rythm ever so often in order to give the music some dynamism. While it's traditionally played with only three panderos, it is not unusual (and truly a rare yet uniquely amazing sight) to see four or more different panderos of varying combinations playing different rythms at once. The sounds of Plena tend to be more up-beat while Bomba is more earthy in nature.