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 Introduction.