there are different kinds.

natural harmonics are when you lightly place your finger over the strings (mainly on the 5, 7 and 12 frets) and pluck the string. it will sound like a chime.

there are pinch harmonics (listen to zakk wylde or pantera's cemetary gates) cant really explain the sound. their usually high pitched. you move your grip up on the pick and put your thumb near the tip of the pick and pick a note and stike the string with the tip and your thumb. i think theres a video on it somewhere on this site. look at the video for more info.