Here is is, stig:

When you put a capo on at second fret, and play a G chord as if you had no capo on, it is an A chord. Because each fret between the nut and the capo raises the pitch a semi-tone. So capo on second fret makes a G an A, C a D, D an E, etc.

Capo on the fifth fret makes a G chord into a C. What the capo actually does, when on second fret, is it presses the strings down firmly on that fret, so that fret becomes the nut, the string length is shorter, and the pitch higher. If it buzzes, the capo isn't tight enough. If its out of tune, the strings were pushed sideways as you put the capo on (the actual nut has grooves to prevent this from happening).

You are not just asking about capoes though, I think, but also about transposing chords. To know how to do this you have to learn to think in numbers rather than letters: in the key of G, the I, IV and V chords are G C and D; in key of D they are D, G and A, and so on. The numbers correspond with which note of the scale, ascending, in that key, is the tonic (the dominant note) of the chord.

You can get charts for transposing, or you can just count them out--remember a major scale goes tone-tone-semitone-tone-tone-tone-semitone.

The good news, though, is that since your bouzouki is tuned the same as a mandolin or a fiddle, you don't really have to use a capo very often unless you like the effect, or the singer you're playing with favours awkward keys like F or B flat, or anything flat. Nothing hard about playing the A key chords for example.

Other reasons you might use a capo are just to get a different sound, that's a little crisper with fewer overtones, to reduce the action on a hard-to-play instrument, or because you don't know the chords for a particular key.

It's a highly useful accessory to have, especially when playing with other people.

Hope this helps.

Willie-O