What is upper structure? Which are the upper structure triads? How are they used?
The answer to this is as long as you want it to be. I'll give you a short one.
Upper structures are normally major or minor triads made up of extended chord tones and their alterations. E.g. your basic chord tones are 1, 3 and 5 - upper structures are usually made up of any of the other extensions: 7, 9, 11, 13, and alterations to them.
Generally, upper structures are used to superimpose a certain tonal quality over a root note while still using a triad format.
Consider having a C shell voicing (root and seven), and playing a D major triad over the top of it, this is an upper structure and creates a Lydian (13#11) sound.
The reason upper structures are used is to simplify an otherwise complicated voicing in your head. On the piano, you play a root note in your left hand and a major or minor triad in your right hand and you could potentially highlight some extreme chord alterations without having to think about the fact that that's what you're playing... Another example would be
Here are other references on this:
Edit: for clarification, I have tried to make it very clear that I am not saying that an upper structure and a slash chord are the same thing, although in performance they are very similar and often can be the same.
On piano, left hand voicings would always include the root and seventh note of the scale to form a shell voicing unless the pianist is playing in an ensemble with other instruments that cover those notes e.g. bass on root note. In this case, the pianist might only play the US triad in the right hand.