In music, the introduction is a passage or section which opens a
movement or a separate piece. In popular music this is often
called an intro. The introduction establishes melodic, harmonic,
and/or rhythmic material related to the main body of a piece.
Introductions may be an ostinato that is used in the following
music, an important chord or progression that establishes the
tonality and groove for the following music, important but
disguised or out of context motivic or thematic material. As
such the introduction may be the first statement of primary or
other important material, may be related to but different from
the primary or other important material, or may bear little
relation to any other material. (Courtesy of Wikipedia)
Introductions or Intros, which occur at the beginning of a song,
are short (usually only several bars) and are used to establish
a songs mood, tempo, and key. If a song's initial chord
progression starts with the "I" chord (as most songs do), you
can use any Turnaround to create an I