An introduction will often involve a short (4 or 8 bars) musical phrase. This "riff" is designed to attract the attention of the listener and help make the song instantly recognizable and memorable.
It may be repeated throughout the song (e.g., during the lead break, or at the end of the song). The melody of the introduction is usually played against the chord progression of the song, or some part of it (e.g., it may be based on the chords used in the verses, chorus or bridge). The riff may also "borrow" the melody of either the verses or chorus.