A tuba is a large brass instrument that you play by blowing into its mouthpiece. In a marching band, the tuba player plays the lowest, deepest notes (and also has to lug around her tuba).
The tuba is the biggest, and among the newest, of all the brass instruments. Tuba players create the low-pitched sound by making a kind of "buzz" with their breath against the mouthpiece and pressing the tuba's valves to form notes. In an orchestra, a tuba player holds the instrument on her lap, while a tuba player in a marching band would use a sousaphone tuba, or helicon, a lighter tuba that can be worn across a shoulder.