Vibrato is a line placed over the staff which indicates the rapid and slight variation of a notes pitch.
It is produced on string instruments by vibrating the fingers of the fretted-hand, and on wind instruments and singers by breath-control.
Figure 1: Three notes with vibrato.
Wide vibrato vibrates the pitch to as wide as a whole tone. This technique is commonly adopted by electric-guitar players.
Figure 2: Three notes with wide vibrato.
Also see Ornaments | Articulations.