Vibrato is a wavering tone obtained by shaking the finger on the string which it is fretting.
It is an effect which helps give extra "life" to notes, especially sustained notes, by changing their pitch. The techniques are not commonly used on the acoustic guitar or general rhythm playing. However, it is extremely important to many styles involving distorted guitar, e.g., rock or metal, even when playing rhythm (though, in that case, vibratos are usually embellishments).
Players of many instruments, including the human voice, use vibrato to help add expression to sustained notes. Vibrato is performed by rapidly bending the string back and forth, causing a modulation in pitch; therefore, all of the information above about bending applies here. A small, subtle vibrato might not require the assistance of other fingers; the fretting finger should be sufficient.
(Courtesy of Wikibooks)