Here is my take on "High Key." Again, this is just my take, feel free to disagree.
First, what high key is NOT is a shot that is overblown, you've just upped the exposure.
Second, high key should be more than just a lot of bright light. The light should be soft. There should be an almost complete absence of shadow. The background should be light. For instance, the photo above by Fotoi is, IMO, a brilliant example of high key. You can get some terrific high key portraits (if the light is oft) that emphasize lips and eyes and eyebrows. The skin texture may look blown out but it's clear that the photo is more than just upping the brightness.
I decided I wasn't going to use the work of others to illustrate my points here. So all the work is mine, it's all at the same location (Little Sahara, in Utah, USA). The first example is high key. I was able to shoot this because a brief cloud appeared, creating soft light.
View attachment 57754Often times it's easier to grasp these concepts (if you think I have a grasp) by comparing the concepts. So let's look at "low key." To me, low key does not mean you just under-exposed the shot (which is what often happens with a sunrise or sunset). Instead, most of the tonal values should be dark. Here is an example of a low key shot from the same setting, only about an hour later with the sun almost setting and I'm shooting the "dark side" of the dune.
View attachment 57757
Finally, what is chiaroscuro? It's an image with high contrast. The tones may not be soft. There may be (in the same picture) extreme light and extreme dark (as long as they aren't blown-out just a black hole). Same general location (ie: a different dune) at sunset:
View attachment 57758My apologies if my "explanation" still leaves you confused. These examples would be easier to illustrate with people/portraits but I'd have a lot of edits to wade through.