Bending is exactly as it sounds: bending the string to the side by pushing it (towards the sixth string) or pulling it (towards the first string), often while a fretted note is ringing. The first three strings are normally pushed, and the others are normally pulled. This is particularly important on the first and sixth strings: you do not want the string to fall off the fretboard. Whether it is pushed or pulled, the currently-sounded note will be raised in pitch. It is likely that nothing bad will happen — other than a strange and probably undesired (and unpredictable) change in pitch, which can still be distastrous during a live performance!
Many aspiring guitarists cannot bend properly. The sound of a bend is more important than how it is actually executed or how it looks, but a bad bending technique usually leads to a bad sound. Your favorite guitarist might bend using just his or her fingertips and you might be inclined to copy this — don't! Your hands can sound every bit as good as your hero's without copying his or her technique. There are two keys to bending properly: proper thumb positioning, and bending with the proper muscles. Do not keep your thumb behind the neck, where it usually is, but bring it up perpendicular to the neck (a position that is normally incorrect, but not in the case of bending). Keep the fingers firm. Do not bend your fingers, but push or pull with your forearm. You will hardly see your forearm move, possibly just see a couple of muscles flex. It will feel awkward at first, but if you can bend with the thumb in the proper position and without bending the fingers, you are probably doing it correctly.
Many guitarists will have trouble bending more than 1/4 step (half a semitone) or perhaps 1/2 step (one semitone) with only one finger, especially on frets close to the nut and on the thinner strings. It is much easier to bend with more than one finger, for instance, with the index finger on the first or second fret and the ring finger on the third, and pushing or pulling with both fingers in order to bend at the third fret. More fingers may be used if this is not enough. It should be possible to bend at least a full step (the pitch difference of two frets) this way
Bending, whether by pushing or pulling the string, shortens the length of the vibrating portion of the string, and thus always raises the pitch of the note. This means it is easier to slide up rather than down in pitch. To create the impression of bending down, the guitarist uses a technique called pre-bending, that is, bending before the string is struck, then releasing the bend (either gradually or quickly, depending on the intended effect).
Bend and release
The ideas of bending and pre-bending can be combined for a "bend and release", that is, striking a note, bending it up, then releasing it as you would with a pre-bend. This will often be perceived as a "bounce" in pitch, especially if played quickly. The reverse is also possible: pre-bend, release, and bend. Repeatedly and steadily bending and releasing is called vibrato. (Courtesy of Wikibooks)