h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

2 questions.
I have never heard a clarinet (I have, along with other 200 intruments at the time, so I cant tell them apart) and I know our good friend Adolph Sax was a clarinet luthier (or I think I know) and he invented the Sax looking for a Bass Clarinet. Now, I made several statments, correct me if I am wrong, but here, really, come my two questions.

1.- Does the Sax sound almost entirelly unlike a clarinete?
2.- What on criket is a Transposition Intrument? I sort of gathered it is an instrument that plays a different note than the one you think it is playing, or rather, the one you think you are playing, or better said, the one you are reading, but that is just teling me what transpositioning is, not what it is, I mean, why was it invented, what purpuse does it serve?

Thanks a bundle.

I got this answer from one of the researchers, any thoughs? http://www.bbc.co.uk/dna/h2g2/classic/F95646?thread=639792&post=7010710#p7010710