Originally Posted by: john828How does this work and what is it exactly (modulation)? I hear that it's the changing of keys, but how does one do that?

Modulation is a using notes or chords from a different key, but still remaining in the original key signature (in other words, without changing completely to that other key signature).

Modulation can be thought of as a "temporary" change of key signature.

A simple example would be playing in the key of A major, using the chords A major, D major & E major (all the notes in those chords are built from notes entirely in the key of A, from the A major scale). Then, you include a C major chord, which is built from notes not in the key of A major. You've just modulated.

Or you could play a whole melody or chord progression in the key of A major, then change the whole thing to the key of E major. This is more commonly called transposition, but sometimes people refer to it as modulation.

These tutorials on improvisation cover modulation.

www.guitartricks.com/tutorial.php?input=876
www.guitartricks.com/tutorial.php?input=491

This tutorial on music theory covers secondary dominants, which are the main music device used to modulate.

www.guitartricks.com/tutorial.php?input=495

This tutorial on the circle of fifths also covers secondary dominants & modulation in a few different genres.

www.guitartricks.com/tutorial.php?input=835

Modulation is a way of making music sound more interesting, less bland. :) It is a common practice of virtually all styles of music from Baroque & Classical to Jazz & Blues to Country, Rock & Pop. Hope this helps. Ask more if necessary! Have fun.