Slide Guitar
Slide Guitar uses a device called a slide that serves as a moveable fret to allow the manipulation of pitch without the limitations that bending and whammy bars (and even whammy pedals) have. It additionally provides a unique tone. It is commonly found in Country and Blues, but has more limited use in many other genres.
Slide Types
Slides come in different forms and materials. Common materials are metal and glass, with each providing different tone. The most common format is a hollow cylinder that you "wear" on one of your left-hand fingers. That is the format we will focus on.
Technique
The slide should be pushed against the strings hard enough to act like a fret. Too loose and the string's vibration will rattle against the slide. Too hard and you'll push the string against the frets, leaving your slides sounding like a noisy chromatic scale. Usually, you'll place the slide over your ring finger or pinky finger. You can also use your middle finger, although this may make things more difficult. The fingers "behind" the slide (index and middle if using your ring finger) should press lightly against the strings - this prevents the portion of the string between the slide and nut from resonating, which can actually transfer energy to the other portion of the slide which will interfere way the string resonates, the same way how picking the strings behind the nut is audible to the guitar pickups.