String bends are notated several different ways in guitar tablature. In the formal guitar tab found in guitar magazines, generally string bends are shown with an upward arrow, accompanied by the number of steps the string should be bent (1/2 step = 1 fret).
In ASCII (text-based) guitar tab, a b is often used to signify a string bend. This b is followed by the fret at which the original note should be bent to. For example, 7b9 would indicate that you should bend the seventh fret until it sounds like the ninth fret.
Sometimes, this target note is included in brackets, like this: 7b(9).
Occasionally, the b is omitted altogether: 7(9).
An r is generally used to indicate a return of a bent note to it's unbent state. For example, 7b9r7 indicates a note on the seventh fret being bent up to the ninth fret, then returned to the seventh fret while the note is still ringing.