The euphonium looks like a small tuba, sounds like a trombone on Valium, is mistakenly referred to as a baritone by too many people, and smells like... well, brass. The greatest and most sonorous of all instruments, the name comes from the Greek euphonos, or 'beautiful sounding', a related word being euphoria (the state in which a euphonium soloist exists during performances). The euphonium has a range extending from the b-flat below the bass clef up through three octaves. The sound of the euphonium can be mistaken by the untrained as that of a tuba, a French horn, a trombone, or even a cornet or trumpet. But the wholesome, low, dark sound of this instrument is unique and beautiful.
Physically the instrument looks like a dwarfed tuba, and is used to cover the tenor tuba parts in orchestral works (such as those of Wagner) as a result of this. The instrument is brass, can have anywhere from three to five valves, and has conical tubing. If the tubing is cylindrical the instrument is a baritone - there is a difference.
Compositions for this beautiful instrument, sadly, are few and far between. Soloists tend to resort to playing trombone, tuba, or bassoon works as a result. Vaughn Williams, Holst, and Sousa regularly included the euphonium in their works, with some of the most beautiful solos for this instrument being found in the Planets suite and the Military Suite in F by Holst.