what are harmonics?

There's a search function on this board that will probably help with questions like this... typing in 'harmonics' brings up three pages of threads. Here's one:

Harmonics

harmonics are cool. thats what they are. Pinch 'em, tap 'em, natural, artificial, but please dont turn into zakk wylde. Tastefully, my friends, tastefully.
(nothing against mr. Wylde, but in my humblest of humble opinions, he uses pinch harmonics too much)

however, natural harmonics almost always sound really nice.