Vibrato
Vibrato is the effect where the pitch of a note is varied rapidly so it rises and falls in a continuous cycle (think: opera singers!). For notes that are sustained, you can get some extra expression into the note using vibrato. On the guitar, the effect is achieved by doing a series of small bends and releases.
Let's say you're playing a note at the 5th fret of the top E string. Pick the note and bend it up in the usual way. Now release the bend and let the string go back to it's usual position, now bend it again, then release again. You need to do the series of bend-release-bend-release smoothly and reasonably quickly. The effect you should get is a continuously varying ("wobbling") pitch. Technically, it is the fluctuation of pitch. An economical way of vibrato is by tingling your finger across the string you are pressing. An impressing vibrato will be will be by beginning it at a slow speed and increasing speed gradually.
Clearly, the depth of the vibrato (how far the pitch varies from it's normal value) and the rate of the vibrato (how quickly the pitch changes) can be varied by bending the note further each time, or by playing the bend-release cycle faster. As a rough guideline:
Vibrato depth: bend the note by a fairly small amount (less than a semitone)
Rate: aim to do a few (3-5) cycles of bend-release every second.