Mariachi is a kind of music that is very popular in Latin America, especially Mexico. A mariachi band has at least five members: violin, trumpet, and three kinds of guitar: regular guitar, vihuela (high pitched), and guitarron (low pitched). Professional mariachi bands, as you might see in a concert, have three or four violin players and at least two trumpets, which makes eight or nine members all together. But smaller groups, like in an authentic Mexican restaurant, might just have one of each.
The name mariachi comes from marriage, because mariachi bands used to play at weddings. This style of music originated in Jalisco, Mexico, but now is popular all across the Southwest United States.