When you bend a string you're adding tension to it, therefore changing (raising) its pitch. A common rule when bending strings is to push the thin E, B & G strings up, but when bending the thick E, A & D strings, pull them down.

Bend Up (push the string up)

Bend Down (pull the string down)


Depending on how much you push or pull the strings will determine what pitch the note becomes. When you play a note on the guitar you're pressing down on the fret. This note is a set, correct pitch and providing the guitar is in tune all other notes on that string will also be in tune as well. When you bend a string you are no longer changing each note by frets but by every increment between those two frets. This means that you are putting the guitar out of tune for a bit before you reach the desired note you're bending to. Because of this when people start learning bends it can sound a little sickly. It has been described to me by one of my lesson as "It sound a bit like a dying cow".