AKA "artificial harmonics"

An electric guitar technique in which the player slightly touches the string AFTER picking a note. The fundamental sound of the note will be effectively cancelled, producing a high-pitched, chimelike harmonic squeal in any position on the fretboard.

Often used to "spice up" rock and heavy metal songs, when overused they are a great way to disguise terrible playing and/or writing.

On the guitar, pinch harmonics is the act of lightly touching a string with the edge of your thumb/palm (picking hand) after plucking it. Similar to natural harmonics except you're not playing off a fret. If you use a lot of distortion and gain it produces a squealing effect.