Guitar Mechanism

I've seen people use thing before, its like a tube that goes around your non struming index finger that plays over the frets. What is it, what does it do?

Hi There-

It's a slide. It can be made of metal, or glass, or porcelain, (or....?) It's used in a style of playing called "Slide Guitar", especially used a lot in rock and country styles. It's a whole other kind of left-hand technique that takes a little getting used to! The sound it creates is long sliding fluid notes or chords that flow into one another.
Hope that helps!
Lisa McCormick - Guitar Tricks Instructor

One of my little party pieces is playing slide (to a modest standard :o ) with whatever is to hand... little jars work pretty well with open tunings...

It's a totally different technique as Lisa says - and if the action on your guitar is low it's going to sound pretty nasty...

Did someone say silde guitar!!!? Awesome! Another opportunity to mention Derek Trucks. Well he's godly and plays slide guitar, so there. Check him out. :D Okay I'm done.

Woops - did I forget to say Blues too? Check out some of the hot lead guitar playing by Bonnie Raitt. Also many more traditional blues players.
It's often used in conjunction with an open tuning on the guitar, too.
-Lisa