The actual mechanics vary between the different makes of capo. For
            instance, I use a Shubb capo which consists of a curved metal bar
            with one "hinged" arm and one pivot arm in a curved "E" shape. The
            capo is placed just behind the fret. The main bar is fitted with a
            rubber sleeve which covers the strings, and the hinged arm fits behind
            the neck. The pivot arm has an adjustable screw which pivots on the
            hinged arm locking the capo in place. The adjustable tension screw
            can therefore be adjusted to fix the capo at different positions on
            the neck without using excessive force which could cause damage. 
          If we place the capo behind the first fret, all the strings have
            been raised by a semitone. If we play a G chord shape, you are really
            playing a G#/AB chord. If we place the capo behind the second fret,
            all the strings have been raised by a tone. If we play a G chord shape,
            we are really playing an A chord. If we place the capo behind the
            third fret, all the strings have been raised by three semitones.If
            we play a G chord shape, we are really playing a A#/Bb chord and so
            on. 
          Can you spot the pattern? If we place the capo at fret "x", whatever
            chord we play will be "x" semitones higher. This principle also applies
            in reverse, so that if we place the capo at fret "x", we play a chord
            "x" semitones lower than the one written. 
          For instance, suppose a song has Eb, AB and Bb chords in it. We could;
            Place the capo at the first fret and play E, A and B chords respectively,
            
            Place the capo at the third fret and play C, F and G chords respectively,
            
            Place the capo at the sixth fret and play A, D and E chords respectively
            etc etc. 
          It is important to remember that any chords and tablature are played
            relative to the position of the capo - for instance, if the capo is
            placed behind the fifth fret, a G chord will written as 320003, even
            though the actual frets are 875558. If you visualise the capo as the
            nut, this approach makes sense, and allows you to think in terms of
            more familiar keys, chord shapes and patterns.