If the director is the god of the band, then the drum major is his jesus. Always a senior who was usually mistreated as a freshman and takes it out on the freshies in the band, but is generally a cool cat to everyone else.
Despite the name, most drum majors actually play trumpet, because playing lead trumpet requires the most musical ability and leadership skills (I was the first low brass player at my school to be drum major ever)
When I was drum major, I told all the freshmen that BFMT ('bout face mark time) in the field show diagrams stood for backflip mark time, because when I was in 9th grade I was told that by seniors.