Bends can be used to gradually approach a note, either up or down. They can be very expressive when used tastefully (just like our arsenal of other legato techniques).
With a few exceptions, bends are either ONE or TWO frets. If you bend 1.5 frets or .5 frets, you will most likely get a horrible sound. Our ears are trained to listen for one of those twelve original notes, and if we don't hear them we cringe. So when you practice bends, make sure you are bending up to exactly one or two frets up. After you release the bend, simply check your work. Play the note you were trying to bend up to and see if it matches. Some people even use a tuner to make sure they bend up to exactly the right pitch!
You can't just make up where you're bending to. When you're in the key of C, and you are playing an F and decide to bend it, it must be a full (two fret) bend. The next note in the scale is G, two frets up. If you did a half bend (one fret) to F#, it would sound awful in the context of the music. In other words, don't just randomly pick a full or half bend! Use your knowledge of scales and your ear to pick the right kind of bend.
Bend the note up first, pick it, and then release it back to normal pitch. Very useful for variety