Modulation is usually brought about by using certain chordss, which are made up of notes common to both the old key and the new one; such chords are known as "pivot chords". The change is solidified by a cadence in the new key.
It is smoother to modulate to some keys than others, because certain keys have more notes in common with each other than others, and therefore more possible pivot chords. Modulation to the dominant (a fifth above the original key) or the subdominant (a fourth above) is relatively easy, as are modulations to the relative major of a minor key (for example, from C minor to E flat major) or to the relative minor of a major key (for example, from C major to A minor; see relative minor/major). These are the most common modulations, although more complex changes are also possible.
Another common form of modulation is using the cycle of fifths:
C - G - D - A - E - B - F# - C# - G# - D# - A# (B flat) - F - C
A sequence, which is the repitition of a short passage at different pitches, may involve modulation, but usually does not. In cases where the key is changed, the sequence is sometimes called a rosalia.
The term "truck driver's gear change" has been used by Siegfried Baboon  to describe the stereotyped use of key changes in popular music, where the whole melody is simply transposed up by an interval to provide an "emotionally uplifting" finale.
Compare with: bitonality and polytonality.
A different, unrelated use of the word modulation in music is found in electronic music, where it can refer to certain methods of altering sounds such as ring modulation (see also modulation).