What is the difference between the C B A and the G clarinet?

In an orchestra, the "normal key" is C. So, if you are playing a C note on a C Clarinet it will sound as a C. On a B(B flat) clarinet, when you play C it will sound B flat. On an A clarinet a C will sound as if A was played and consequently on a G clarinet a C will sound as a G.

So for the clarinets B, A and G, music needs to be transposed (changed), so that we play the correct notes. This is done so we can switch between different clarinets. Otherwise we would need to learn different fingerings for all the different clarinets. The C clarinet is not transposed and we play it as it is.

I hope this makes sense... :)