The following video discusses the bending of guitar strings. There are a few different styles of bending. There are single bends, which can range from 1/8th to a full step or there are vibrato bends. Vibrato are much more dramatic and used for those purposes mainly, especially in improvisation and solos. On the other hand, more exact bends are actually used as specific notes in guitar songs.
Read the full article here:
Bending for beginners. Nice.