The guitar bend is a technique that allows you to change the pitch of a string, after it has already been picked by bending it up (higher pitch) or down (to a lower pitch).

You should bend with your entire wrist because a single finger alone may not be strong enough to bend the string as far as you want.
If you use a printed tablature from a book or magazine, the bend is denoted there as a curved arrow pointing upward or downwards (depending on the direction of the bend) and a notation to tell you how much to bend (for example: ‘1/2′, ‘1′, ‘half’, or ‘full’).
On an ASCII tablature, a bend is notated by the letter ‘b’. The following example instructs you to pick the E string while holding the 7th fret and bending it up by 1/2 a tone, to make it sound like the 8th fret.

E||------7b(8)---
B||--------------
G||--------------
D||--------------
A||--------------
E||--------------
The following video demonstrate the above example:

A variation to the regular bend is the bend-and-release. In this technique you first bend the string up and than release it. For instance, the notation: “7b(8)r(7)” tells you to pick the string while pressing the 7th fret, bend it up to sound like the 8th fret. Then, without picking again bend it back to sound like the original note.