The guy above pretty much nailed it. It's worth noting that professional (DCI-sanctioned) drum corps have people aged 14 to 21, and consist of an entire summer of long hours of practice to the point that they kick ass. They go to lots of competitions, the most important of which is Drum Corps International, or DCI. Some of the most well-known corps are the Cavaliers (my favorite, since my best friend played in the pit for two years), the Santa Clara Vanguard, Blue Devils, Boston Crusaders, Phantom Regiment, and Madison Scouts. Corps music is frikking awesome, and each corps has its own style of music that it traditionally plays.