Below is an excerpt from an excellent article by Stanley Jordan about Touch-Style Technique or two-handed tapping. Follow the link to read the whole article.
Excerpted from: http://www.stanleyjordan.com/Technique/starting.html
"STANLEY JORDAN has employed this revolutionary approach to guitar since 1976.
This article is an updated version of "Stanley Jordan On Two-Handed Tapping," which
appeared in the July 1984 issue of Guitar Player Magazine
Getting Started with the Touch Technique
by Stanley Jordan
THE TOUCH STYLE, OR TWO-HANDED TAPPING TECHNIQUE can provide limitless possibilities for exploration on the guitar. The earliest documented guitarist using this approach was Jimmy Webster in the 1950s. It has now begun to enjoy considerable use among guitarists. The essence of tapping is this: By hammering the string against the fretboard with your finger, you can produce a note with one hand. You don't need to pluck or strum, because the impact of the string hitting the fret causes the string to vibrate. Either hand works, and you can even use both hands tapping simultaneously on the fingerboard, performing independent parts.
Producing the sound in this way is easy. But mastering its awesome and unexpected possibilities is another matter! It gives you a level of musical and orchestral complexity previously possible only on keyboard instruments. You can create bass and chord accompaniment to your own leads as a self-contained soloist. You can also perform complex counterpoint, such as Bach two and three-part inventions. With a band, you can use your hands together to play leads with undreamed-of speed and agility."
Read More (Link to article on Stanley Jordan's website):