No, that's not a Tuba; it's a Euphonium (Greek: "beautiful sound"). Don't know what it is? Well it's a Brass instrument, so it should be listed as such in the encyclopedia on this site, so, you can look it up there. Surprised? No results.
When I first got into this instrument, I had thoughts about playing it for a living and had eventually, I dreamed of playing it in a professional orchestra; a dream that was quickly shattered, because it is not considered an orchestral instrument. But why not?
It is a fairly young instrument, but nonetheless, very usefull and very versatile. It's range is identical to that of a Trombone, but can - on a 4-valve model - extend to notes below the Trombone's range (without the use of pedaling): A fourth, to be exact. It is a member of the brass section in a concert band and has earned the nickname, "Cello of the band". It is also a very significant and often important member of the brass band, though not common in brass quintets. It can be read either in concert pitch or transposed in Bb. Is all of this not enough to account for it's youth and lack of exposure?