The tuba is the largest and lowest pitched brass instrument.
Sound is produced by vibrating or "buzzing" the lips into a large cupped mouthpiece.
It is one of the most recent additions to the modern symphony orchestra, first appearing in the mid-19th century, when it largely replaced the ophicleide.
Tuba is Latin for trumpet or horn.
The horn referred to would most likely resemble what is known as a baroque trumpet.