Players of many instruments, including the human voice, use a vibrato to help add expression to sustained notes. Vibratos are fairly easy to execute and add a lot of style and depth to your playing.
Vertical vibrato is performed by rapidly bending the string back and forth, causing a modulation in pitch.
Horizontal vibrato is performed by rapidly sliding your finger up and down a fret, but this techinque is more commonly used by violin players.
The Vibrato bar
The effects of bending and vibrato can be produced in a completely different way by manipulating the guitar's vibrato bar (often called a tremolo bar after Fender, its inventor, who misnamed it) if the guitar has one, Guitar icon Jimi Hendrix is a good example of deploying this style. However, using the vibrato as a substitute for fretting-hand bending is not good practice; it is best used for very heavy bends or vibratos. It is more difficult to be subtle with a vibrato bar, and it is usually a bit out of the way for the picking hand to reach, making it harder to use. In short, while in some cases which style of bending or vibrato is used is a matter of taste, the two techniques are not interchangeable and are used for different effects. The vibrato bar, also known as a whammy bar, is often time used to create very fast vibratos. The sound of a vibrato can also be mimicked by a whammy pedal or a wah pedal.
Performing A Vibrato
Performing a vibrato is much like performing many bends over and over again in a very fast tempo. Another technique used with peforming a virbato is using an "artificial harmonic" along with the vibrato.