Modulation

The most common type of modulation is to change to the key which has 1 note different.  For example, from C major to G major:

C, D, E, F, G, A, B to G, A, B, C, D, E, F#

We subconsciously sense the change of the team leader (tonic) from C to G.  Althernatively we could change from C major to F major which contains all notes from C except B falls to Bb.  The emotional impact usualy lasts for a short duration (10 of seconds).

Composer usually modulate a phrase to key with many different notes to inject more interest.  For example from C major to E major.  Changing the key a semitone above or below brightens up the music.  This is known as truck driver's modulation.

Changing from a major key to minor key add further interest to the mood.  Modulate too frequent may confuse the audience while too little makes the music bland and predictable.