Help please!

Can anybody explain to me in idiot terms what happens when I place a capo on a bouzouki (tuned GDAE)? I understand it changes the key, but when I play a G chord with no capo it sounds G, so when I put the capo on (for example) the second fret and play the G chord it doen't sound G does it, but another chord?

So . . . if I'm playing a tune in A, I put my capo on the second fret and play what chords? If the tune is chorded in D on the sheet music, do I play the D chords or do I have to learn a complete new set of chords?

If this seems elementary, it's because it is and i can't find a decent explanation of what a capo actually does.

Cheers!

stigWeard